In a race of passed pawns on opposite wings, the defender must stop the enemy pawn with the rook from behind or in front at exactly the right moment, and keep the king near the enemy pawn.
Both sides have a passed pawn, White on a5 and Black on h5. White's rook is already behind its pawn on a1, which is the ideal placement, while Black's rook on b2 is temporarily cutting the white king off along the second rank. The question is how Black stops the a-pawn.