I am on page 118 of "Play winning Chess" and not we are looking at pawn structure. Exploiting weak pawns can be an excellentto employ.
Single pawns or grops of pawns that are separated from other pawns by at least one file are pawn Islands.
It has been said whoever has the most pawn islands looses. So It only takes one file to qualify as a pawn island. So some are weaker than others.
A pawn that makes a capture and ends up on the same file but in front of another pawn of the same color is called doubled. They are considered weak since they do not have the mobility of other pawns. they can be useful on a center file. In the below example we see Doubled pawns on c3 and c4. c4 is what the book says is most vulnrable but I see that the knight is supporting it. But I guess then he can't do anything else. Let's play through it and see what Yasser says.