It's true that marijuana is notorious for making users crave junk food - Doritos, potato chips, a plate of
loaded nachos, etc. (I saw a funny incident on an episode of
COPS where, during a traffic stop, the police officer smelled a strong marijuana odor. "Have you been smoking pot, sir?" he asked. "Absolutely not," the glassy-eyed driver replied. "Oh, really? Because I'm getting the munchies just standing here," the cop scoffed.) But being hungry for snack-type food is also commonly called "the munchies" even when the person is stone cold sober: "I shouldn't be hungry since we just ate dinner an hour ago, but I've got the munchies. Do we have any microwave popcorn in the cupboard?"
"Munch" as a verb can also be used to describe the actual act of eating:
"I didn't have lunch today; I'm on a new diet, so I just munched carrot and celery sticks all afternoon."
"The giant panda spends most of his waking hours munching bamboo shoots."
Usually "munching" describes eating rather casually, over a more prolonged period than just a meal. For example, munching on popcorn while watching a movie, or munching on cheese and crackers during a book club meeting.