user1839: It doesn't kick the bishop off that diagonal, and playing ...b5 later puts the bishop on another great diagonal and is supposedly bad for black (an explanation of this would be great too). So why is 3...a6 in the main line for black?
[FEN ""]
1.e4 e5 2.Nf3 Nc6 3.Bb5 a6


user70: Here's a common variation:

[FEN ""]
1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4 b5 5. Bb3


The white Bishop is on a decent square.  It attacks a center square (d5), the weak f7 pawn, and the square to which the Black King will likely retire (g8).  However, that's all this Bishop can do.  Its influence is limited to 6 squares and it has little mobility.  Black has gained space on the Queen side.  Bb7 is a nice move.  Black can be thinking about playing c5 followed by Qb6, or Bb5.
In short, the position is very flexible for Black.  Watch that f7 square, though!

user2001: The point of a7-a6 is to enter variations where white's light squared bishop will be kicked back to c2 and black will grab space in the center with c7-c5. See for instance this variation:
[FEN ""]
1.e4 e5 2.Nf3 Nc6 3.Bb5 a6 4.Ba4 Nf6 5.O-O b5 6.Bb3 Be7 7.d3 O-O 8.c3 d6 9.Nbd2 Na5 10.Bc2 c5

It is a popular variation for black and black has plenty of ideas. A complicated battle lies ahead. The short term point of a7-a6 is to kick away the bishop from the a4-e8 diagonal. The long term point is to get these types of positions where both sides have plenty of ideas and chances to fight for the initiative.

user1124: The idea of 3. .. a6 is to force white player to make a decision about own black Bishop already in this part of the game. 
For example:
[FEN ""]
1.e4 e5 2.Nf3 Nc6 3.Bb5 a6 4.Bc6 

or 
[FEN ""]
1.e4 e5 2.Nf3 Nc6 3.Bb5 a6 4.Ba4 

Its two different variations with tons of theory in each of it.
If you want the evaluation of 3. ... a6 move - it is neutral one, not aggressive.
In the same category is for example 3. ... d6
[FEN ""]
1.e4 e5 2.Nf3 Nc6 3.Bb5 d6 

Not aggresive not mean weak - it is a solid one.
In contrast to 3. ... a6 or 3. ... d6
moves like 3... f5:
[FEN ""]
1.e4 e5 2.Nf3 Nc6 3.Bb5 f5

3... Nf6:
[FEN ""]
1.e4 e5 2.Nf3 Nc6 3.Bb5 Nf6

3... Nd4:
[FEN ""]
1.e4 e5 2.Nf3 Nc6 3.Bb5 Nd4

forces the combination game.
To show the example of passive move in this position is
3... Nge7:
[FEN ""]
1.e4 e5 2.Nf3 Nc6 3.Bb5 Nge7

waiting for white action even creating some minimal troubles for own black bishop here.
To summarise it:
the purpose of 3. ... a6 in Ruy Lopez is to become more information about enemy black Bishop and as a result the whites plans for this game.

user1824: Both diagonals a4-e8 and a2-g8 are good for white's bishop. Ruy-lopez strategy is to pressure on black's position by a4-e8 diagonal. Ruy-lopez claims this diagonal is more dangerous and annoying for black. Indirect pressuring on black's king and then on the knight on c6 is the idea.
The pawn on d7 is going to advance and after that the knight on c6 which is protector of e5 will be pinned by the bishop. It can reduce the speed of black's developing. After a6 white shall decide to exchange or backward the bishop. After backward, black has the ability to play b5 and escape from pinning.
Also, playing ruy-lopez without a6 is possible but history shows, it's better to play the main variant.

user1856: After ...a6,  white has to make a choice to either play Bxc6 or Ba4. If Bxc6 then black has enough resources to protect his e5 pawn and maintain a stable position (if 4.Bxc6 dxc6 5.Nxe5?! Qd4 and black will regain his pawn with good position. And if white doesn't play 5.Nxe5 then black has the bishop pair and simple development; black aims to prove that Bxc6 was premature).
Thus white tries Ba4 instead to try to maintain some edge, he still threatens Bxc6 but waits until he protects his own e4 pawn first before doing so. Seeing all this, that's the reason why black plays ...a6 immediately. Black can play ...b5 when necessary to help protect his e5 pawn when time comes. Yes the bishop goes to another good diagonal, but black doesn't lose any tempo with ...a6 and ...b5 so it's even ground. If black plays ...a6 later instead, white can -under good circumstances-  immediately play Bxc6 (trying to win the e5 pawn, which is what he was threatening to do anyway) but without losing a tempo by retreating to a4 first.

user291: Playing ...b5 later is not bad for black. Playing it immediately isn't so great, but Black almost always plays ...b5 at some point (unless White plays Bxc6 first!).
The main reason to play ...a6 early is that you can now play ...b5 any time you want to if you decide that you don't White to take the knight on c6. If you don't play ...a6, and then later decide that you want to kick the bishop away and start by playing ...a6, White can go ahead and play Bxc6. If you've already played ...a6, you can go straight to ...b5 and White doesn't get a chance to capture your knight in response.

1839: <p>It doesn't kick the bishop off that diagonal, and playing ...b5 later puts the bishop on another great diagonal and is supposedly bad for black (an explanation of this would be great too). So why is 3...a6 in the main line for black?</p>

<pre><code>[FEN ""]
1.e4 e5 2.Nf3 Nc6 3.Bb5 a6
</code></pre>
 70: <p>Here's a common variation:</p>

<pre><code>
[FEN ""]
1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4 b5 5. Bb3
</code>
</pre>

<p>The white Bishop is on a decent square.  It attacks a center square (d5), the weak f7 pawn, and the square to which the Black King will likely retire (g8).  However, that's all this Bishop can do.  Its influence is limited to 6 squares and it has little mobility.  Black has gained space on the Queen side.  Bb7 is a nice move.  Black can be thinking about playing c5 followed by Qb6, or Bb5.</p>

<p>In short, the position is very flexible for Black.  Watch that <code>f7</code> square, though!</p>
 2001: <p>The point of a7-a6 is to enter variations where white's light squared bishop will be kicked back to c2 and black will grab space in the center with c7-c5. See for instance this variation:</p>

<pre><code>[FEN ""]
1.e4 e5 2.Nf3 Nc6 3.Bb5 a6 4.Ba4 Nf6 5.O-O b5 6.Bb3 Be7 7.d3 O-O 8.c3 d6 9.Nbd2 Na5 10.Bc2 c5
</code></pre>

<p>It is a popular variation for black and black has plenty of ideas. A complicated battle lies ahead. The short term point of a7-a6 is to kick away the bishop from the a4-e8 diagonal. The long term point is to get these types of positions where both sides have plenty of ideas and chances to fight for the initiative.</p>
 1124: <p>The idea of 3. .. a6 is to <strong>force</strong> white player to make a <strong>decision about</strong> own <strong>black Bishop</strong> already in this part of the game. </p>

<p>For example:</p>

<pre><code>[FEN ""]
1.e4 e5 2.Nf3 Nc6 3.Bb5 a6 4.Bc6 
</code></pre>

<p>or </p>

<pre><code>[FEN ""]
1.e4 e5 2.Nf3 Nc6 3.Bb5 a6 4.Ba4 
</code></pre>

<p>Its two different variations with tons of theory in each of it.</p>

<p>If you want the evaluation of 3. ... a6 move - it is neutral one, not aggressive.
In the same category is for example 3. ... d6</p>

<pre><code>[FEN ""]
1.e4 e5 2.Nf3 Nc6 3.Bb5 d6 
</code></pre>

<p>Not aggresive not mean weak - it is a solid one.</p>

<p>In contrast to 3. ... a6 or 3. ... d6</p>

<p>moves like 3... f5:</p>

<pre><code>[FEN ""]
1.e4 e5 2.Nf3 Nc6 3.Bb5 f5
</code></pre>

<p>3... Nf6:</p>

<pre><code>[FEN ""]
1.e4 e5 2.Nf3 Nc6 3.Bb5 Nf6
</code></pre>

<p>3... Nd4:</p>

<pre><code>[FEN ""]
1.e4 e5 2.Nf3 Nc6 3.Bb5 Nd4
</code></pre>

<p>forces the combination game.</p>

<p>To show the example of passive move in this position is
3... Nge7:</p>

<pre><code>[FEN ""]
1.e4 e5 2.Nf3 Nc6 3.Bb5 Nge7
</code></pre>

<p>waiting for white action even creating some minimal troubles for own black bishop here.</p>

<p><strong>To summarise it:</strong></p>

<p>the purpose of 3. ... a6 in Ruy Lopez is to become more information about enemy black Bishop and as a result the whites plans for this game.</p>
 1824: <p>Both diagonals <code>a4-e8</code> and <code>a2-g8</code> are good for white's bishop. Ruy-lopez strategy is to pressure on black's position by <code>a4-e8</code> diagonal. Ruy-lopez claims this diagonal is more dangerous and annoying for black. Indirect pressuring on black's king and then on the knight on <code>c6</code> is the idea.</p>

<p>The pawn on <code>d7</code> is going to advance and after that the knight on <code>c6</code> which is protector of <code>e5</code> will be pinned by the bishop. It can reduce the speed of black's developing. After <code>a6</code> white shall decide to exchange or backward the bishop. After backward, black has the ability to play <code>b5</code> and escape from pinning.</p>

<p>Also, playing ruy-lopez without <code>a6</code> is possible but history shows, it's better to play the main variant.</p>
 1856: <p>After ...a6,  white has to make a choice to either play Bxc6 or Ba4. If Bxc6 then black has enough resources to protect his e5 pawn and maintain a stable position (if 4.Bxc6 dxc6 5.Nxe5?! Qd4 and black will regain his pawn with good position. And if white doesn't play 5.Nxe5 then black has the bishop pair and simple development; black aims to prove that Bxc6 was premature).</p>

<p>Thus white tries Ba4 instead to try to maintain some edge, he still threatens Bxc6 but waits until he protects his own e4 pawn first before doing so. Seeing all this, that's the reason why black plays ...a6 immediately. Black can play ...b5 when necessary to help protect his e5 pawn when time comes. Yes the bishop goes to another good diagonal, but black doesn't lose any tempo with ...a6 and ...b5 so it's even ground. If black plays ...a6 later instead, white can -under good circumstances-  immediately play Bxc6 (trying to win the e5 pawn, which is what he was threatening to do anyway) but without losing a tempo by retreating to a4 first.</p>
 291: <p>Playing ...b5 later is not bad for black. Playing it immediately isn't so great, but Black almost always plays ...b5 at some point (unless White plays Bxc6 first!).</p>

<p>The main reason to play ...a6 early is that you can now play ...b5 any time you want to if you decide that you don't White to take the knight on c6. If you don't play ...a6, and then later decide that you want to kick the bishop away and start by playing ...a6, White can go ahead and play Bxc6. If you've already played ...a6, you can go straight to ...b5 and White doesn't get a chance to capture your knight in response.</p>
