When you crush something, you break it into tiny pieces or crumple it. If you crush the cans in your recyclable bin, you'll make more room for more.
A train can crush a car that's stalled on the tracks, and a person can crush an insect between her fingers. A figurative meaning of crush is to subdue or stop, like when a military dictator's forces crush a rebel uprising. You can also use crush as a noun to mean "an overwhelming number," like a crush of bodies on a packed subway car, or to describe infatuation, like your crush on your attractive neighbor, who also can be called your crush.