user31998: 
Quoting Irving Chernev:

A brilliant sacrifice which must be accepted. Refusing the Pawn means that Black could never free himself by … d5. It would also enable
White to play 16 ♘c3 next move (attacking the Queen) and thus gain
time for 17 ♘d5, establishing a strongly supported outpost.
Chernev, Irving. The Most Instructive Games of Chess Ever Played (p.
37).

I have read this multiple times in this book, but I don't quite grasp why an outpost guarded by two pawns is considered stronger than an outpost guarded by only one?

user12629: An outpost guarded by two pawns is stronger for two reasons.
First, if the piece is guarded by two pawns, that means it is not possible for the opponent to win a pawn by attacking it with pieces. If a knight on d5 was supported only by a pawn on e4, then Black could play something like Nxd5 exd5 Bxd5. But with two pawns guarding the outpost, Black can't win material no matter how many pieces are attacking d5; the best they can get is an even trade.
Second, an outpost can be undermined by attacking the supporting pawn. If there are two pawns supporting it, this obviously becomes more difficult to do.

31998: <p><a href="https://i.stack.imgur.com/W39La.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/W39La.png" alt="enter image description here" /></a></p>
<p>Quoting Irving Chernev:</p>
<blockquote>
<p>A brilliant sacrifice which must be accepted. Refusing the Pawn means that Black could never free himself by … d5. It would also enable
White to play 16 ♘c3 next move (attacking the Queen) and thus gain
time for 17 ♘d5, establishing a strongly supported outpost.</p>
<p>Chernev, Irving. The Most Instructive Games of Chess Ever Played (p.
37).</p>
</blockquote>
<p>I have read this multiple times in this book, but I don't quite grasp why an outpost guarded by two pawns is considered stronger than an outpost guarded by only one?</p>
 12629: <p>An outpost guarded by two pawns is stronger for two reasons.</p>
<p>First, if the piece is guarded by two pawns, that means it is not possible for the opponent to win a pawn by attacking it with pieces. If a knight on d5 was supported <em>only</em> by a pawn on e4, then Black could play something like Nxd5 exd5 Bxd5. But with two pawns guarding the outpost, Black can't win material no matter how many pieces are attacking d5; the best they can get is an even trade.</p>
<p>Second, an outpost can be undermined by attacking the supporting pawn. If there are two pawns supporting it, this obviously becomes more difficult to do.</p>
