A weak pawn is a pawn that cannot be defended by another pawn and therefore becomes a permanent target for the opponent’s pieces.
A pawn is considered weak when no neighbouring pawns can protect it — either because there are none on the adjacent files, or because they are blocked or too far away. The most common cases are the isolated pawn (alone on its file with no neighbours), the backward pawn (unable to advance without being lost, left behind by its neighbours), and the doubled pawn (two pawns on the same file, one of which cannot be supported by another pawn). In all these cases, the opponent can place a piece in front of or facing the weak pawn and apply lasting pressure.
In practice, identify your weak pawns as soon as you leave the opening. If you have one, use a rook or another active defender to cover it — but more importantly, look for dynamic compensation: an open file, an attack on the opponent’s king, or piece activity that offsets the structural weakness. If it is your opponent who has a weak pawn, fix it in place (prevent it from advancing), press it with your pieces, and steer the game towards an endgame where the weakness will prove decisive.