The rook on the seventh and an active king outweigh several pawns. Capablanca gave up two pawns to bring his king to f6, where it combined with the rook and the g-pawn to create unstoppable threats.
One of the most quoted rook endings in chess literature. White's rook is on the seventh rank, Black's king is stuck on f8 and the g5-pawn is about to advance. White is only equal in pawns and the c3-pawn hangs, but Capablanca saw that material is secondary when the king can join the attack.