When you crunch something, you grind it into tiny pieces, often with your teeth. You can also crunch whole spices into smaller bits with a mortar and pestle.
A horse might crunch an apple, and on a summer day you might drink a glass of iced tea and crunch the ice between your teeth. You could also describe the sound of gravel underneath the wheels of a car or feet walking in fresh snow as a crunch. A figurative crunch is a deadline or crisis: "When it came to the crunch, I just voted for the person who would do the least harm."