user703: I can not understand what the following numbers mean:

Gligorić had the following record against the world champions he
  played against: Max Euwe +2 -0 =5, Mikhail Botvinnik +2 −2 =6, Vasily
  Smyslov +6 −8 =28, Tigran Petrosian +8 −11 =19, Mikhail Tal +2 −10
  =22, Boris Spassky +0 −6 =16, Bobby Fischer +4 −7 =8, Anatoly Karpov +0 −4 =6 and Garry Kasparov +0 −3 =0.

I took them from here. I do understand that it tracks statistics against a particular opponent in all games, but what is this strange mathematics mean: +8 −11 =19?

user-1: +8 -11 =19 means: Won 8, lost 11 and drew 19

user2042: 
but what is this strange mathematics mean: +8 −11 =19?

This is not mathematical notation. The three symbols are not there so you can add or subtract numbers. They mean the following: 

+ is the number of victories; 
- is the number of defeats; 
= is the number of draws player achieved against an opponent; 

So in the following example:

Gligorić had the following record against the world champions he played against: Max Euwe +2 -0 =5

Gligoric had 2 victories, no defeats and five draws against Max Euwe. 
Against Fisher he had +4 -7 =8, meaning he won 4 games against him, lost 7 and drew 8.

703: <p>I can not understand what the following numbers mean:</p>

<blockquote>
  <p>Gligorić had the following record against the world champions he
  played against: Max Euwe +2 -0 =5, Mikhail Botvinnik +2 −2 =6, Vasily
  Smyslov +6 −8 =28, Tigran Petrosian +8 −11 =19, Mikhail Tal +2 −10
  =22, Boris Spassky +0 −6 =16, Bobby Fischer +4 −7 =8, Anatoly Karpov +0 −4 =6 and Garry Kasparov +0 −3 =0.</p>
</blockquote>

<p>I took them from <a href="http://en.wikipedia.org/wiki/Svetozar_Gligori%C4%87#Lifetime_scores_against_world_champions">here</a>. I do understand that it tracks statistics against a particular opponent in all games, but what is this strange mathematics mean: <strong>+8 −11 =19</strong>?</p>
 -1: <p><strong>+8 -11 =19</strong> means: <strong>Won 8, lost 11 and drew 19</strong></p>
 2042: <blockquote>
  <p>but what is this strange mathematics mean: +8 −11 =19?</p>
</blockquote>

<p>This is not mathematical notation. The three symbols are not there so you can add or subtract numbers. They mean the following: </p>

<ul>
<li><code>+</code> is the number of victories; </li>
<li><code>-</code> is the number of defeats; </li>
<li><code>=</code> is the number of draws player achieved against an opponent; </li>
</ul>

<p>So in the following example:</p>

<blockquote>
  <p>Gligorić had the following record against the world champions he played against: Max Euwe +2 -0 =5</p>
</blockquote>

<p>Gligoric had 2 victories, no defeats and five draws against Max Euwe. </p>

<p>Against Fisher he had +4 -7 =8, meaning he won 4 games against him, lost 7 and drew 8.</p>
