Tarrasch's rule: rooks belong behind passed pawns. A rook behind its own pawn gets stronger as the pawn advances, while a rook in front of or beside it loses room as the pawn moves up.
The black rook attacks the a5 pawn. Defend the pawn from the right place with your rook.
The rule also works for the defender: the best way to stop an enemy passed pawn is usually to put your rook behind it.