IV crush is the rapid deflation of implied volatility once a scheduled event, most commonly earnings, resolves. The event premium that built up in the spanning expiration vanishes overnight, repricing options lower even when the stock moves.
Crush is why buying options into earnings can lose money on a correct directional call: the stock moves, but less than the inflated premium required. The same mechanism is what event-volatility structures harvest, selling the expiration that will crush against one that will not, gated on the event premium actually being rich relative to the stock's realized earnings history.