A backward pawn is a static weakness because it is stuck on a square where it cannot be defended by other pawns and cannot advance without being captured. The square directly in front of a backward pawn is typically a strong outpost for the opponent, since no pawn can challenge a piece placed there.
The most common example is a backward pawn on d6 in the Sicilian Defense. Black's d-pawn cannot advance to d5 because White controls that square, and the d6-pawn becomes a target. White often places a knight on d5, establishing a powerful outpost directly in front of the backward pawn.
Dealing with a backward pawn requires either organizing a break to advance it (eliminating the weakness), exchanging it off, or generating enough counterplay elsewhere to compensate. When playing against a backward pawn, the strategy is to occupy the square in front of it and pile up pressure with pieces, particularly rooks on the open or semi-open file.