Philidor Defence (Rook Endgame)
The Philidor is the key drawing method when you're defending a rook endgame a pawn down. You hold your rook on your third row until the enemy pawn advances, then swing it behind the king to check endlessly. Learn it and you'll save many 'lost' rook endings.
Source for further study: Dvoretsky, Endgame Manual, ch. 7.