In chess, "crushing" describes a position in which one player is not only "winning," but has such a large advantage that any number of of different moves can be made and still maintain the advantage. When one player achieves a crushing position, the other player will usually concede defeat and resign the game.
"Crushing" differs significantly from mere "winning" because a position described as "winning" often depends on the player with the advantage finding an only move or one of a few specific moves to maintain the advantage, where as in a crushing position almost any move will be fine.
Chess advantages are estimated in terms of the approximate number of pawns they are worth. In modern computer evaluation a player is typically said to be crushing if they have an advantage estimated at at least five pawns or greater. A player with an advantage of less than around five pawns worth is typically said to be merely "winning" or even just "better."