Sometimes capturing an opponent's piece with one of your

pawns will result in two of your pawns being right behind one another.

These pawns are considered "doubled," which is not in your best advantage.

The primary reason is their lack of mobility:

first the front pawn must move, then the rear pawn,

then the front, etc. This is good to know when you want to cause

the other player to double up his pawns, causing the same havoc for him.