When a player is losing but can deliver an unending series of checks — usually with the queen, sometimes a rook — the game is drawn because neither side can make progress and the position repeats. The defending king simply cannot find a square that escapes the checks without allowing a worse outcome.
Perpetual check is the great equalizer of sharp positions: a player who has sacrificed material for an attack will often settle for a draw by perpetual if the full point slips away. Recognizing perpetual-check patterns lets you both rescue lost positions and avoid letting a winning game evaporate into a draw.