Backward pawn
A backward pawn cannot be guarded by neighboring pawns and cannot safely advance because the square in front of it is controlled by the opponent.
The pawn sits behind its neighbors. The hole in front of it is a square the opponent can occupy. You spend pieces defending a pawn that cannot easily step forward.
In the Open Sicilian, d6 is the usual backward pawn. White’s e4 pawn (and a knight on d4 or b5) makes …d5 hard to achieve. Until that break works, Black’s queen and rook often stay tied to d6.
The cure is the pawn break that frees it — here, …d5 — or a piece blockade swap that trades the weak pawn off. Sitting behind it forever is how the weakness is milked.