To munch is to eat something in a loud and enthusiastic way. If you can't stand listening to your brother munch his toast in the morning, you might decide to take your breakfast to go.
When you eat something steadily, especially if it's something crunchy, you munch. You might munch on carrot sticks for a snack and then munch a grilled cheese sandwich for lunch. You can also take a munch, or a bite, out of something. Munch is an imitative word, its sound echoing the actual sound of munching, but it may also have a connection to the Old French mangier, "to bite or chew."