Isolated Pawns

A pawn is isolated when there is no friendly pawn on the file immediately to its left or right. Since pawns can only defend each other diagonally, from a neighboring file, an isolated pawn can never be defended by another pawn -- only by pieces. That makes it a permanent long-term liability: pieces have better things to do than babysit a single pawn for the rest of the game.

Isolated pawns aren't automatically bad. An isolated pawn in the center (the famous "isolated queen's pawn", or IQP) often gives the side that has it extra space and open lines for its pieces, which can be worth more than the structural weakness -- especially with queens and many pieces still on the board. But as pieces get traded off and the endgame approaches, the isolated pawn's weakness tends to matter more and more, because there are fewer pieces left to protect it and the opponent's king can attack it directly.

The file the isolated pawn stands on is also permanently half-open for the opponent (since there's no pawn on it to challenge from the sides), which is exactly the kind of file an enemy rook wants to sit on, aiming straight at the weak pawn.