user13147: John Nunn mentions the following in Understanding Chess: Move by Move (2002):

Pawn weaknesses vary in severity according to two factors: 1) whether there is any possibility of liquidating the weakness, ...
As an example of the first factor, a backward pawn may not be a real weakness at all if it is impossible to stop the pawn advancing aside another pawn.

How do these two statements tie hand-in-hand?

user8300: Normally a backward pawn is considered a potential weakness since it cannot be defended by other pawns where it stands, and if it's impossible to move the pawn somehow it can become a very serious weakness indeed. But if the backward pawn can sidestep any threat against it by moving forward it may not even be an issue. In many cases, a backward pawn will come under attack by an enemy pawn if it were to move forward, and this is probably what John Nunn meant in the provided text; the "weak" pawn is pushed forward and will be "liquidated" against the enemy pawn.

user825: The book is right. In chess, nothing is a weakness if it can't be exploited. In the following world championship game, Black had a backward pawn on d6. However, it was not a weakness.
Note b3, Rad8, Red2, d5!, the d5 push will liquidate everything. That was why Carlsen exchanged the pawn with his c4 pawn immediately.  

http://www.chessgames.com/perl/chessgame?gid=1937912



13147: <p>John Nunn mentions the following in <em>Understanding Chess: Move by Move (2002)</em>:</p>
<blockquote>
<p>Pawn weaknesses vary in severity according to two factors: 1) whether there is any possibility of liquidating the weakness, ...</p>
<p>As an example of the first factor, a backward pawn may not be a real weakness at all if it is impossible to stop the pawn advancing aside another pawn.</p>
</blockquote>
<p>How do these two statements tie hand-in-hand?</p>
 8300: <p>Normally a backward pawn is considered a potential weakness since it cannot be defended by other pawns where it stands, and if it's impossible to move the pawn somehow it can become a very serious weakness indeed. But if the backward pawn can sidestep any threat against it by moving forward it may not even be an issue. In many cases, a backward pawn will come under attack by an enemy pawn if it were to move forward, and this is probably what John Nunn meant in the provided text; the "weak" pawn is pushed forward and will be "liquidated" against the enemy pawn.</p>
 825: <p>The book is right. In chess, nothing is a weakness if it can't be exploited. In the following world championship game, Black had a backward pawn on d6. However, it was not a weakness.</p>

<p>Note b3, Rad8, Red2, d5!, the d5 push will liquidate everything. That was why Carlsen exchanged the pawn with his c4 pawn immediately.  </p>

<blockquote>
  <p><a href="http://www.chessgames.com/perl/chessgame?gid=1937912" rel="noreferrer">http://www.chessgames.com/perl/chessgame?gid=1937912</a></p>
</blockquote>

<p><a href="https://i.stack.imgur.com/zn9zn.png" rel="noreferrer"><img src="https://i.stack.imgur.com/zn9zn.png" alt="enter image description here"></a></p>
