A snack is a small amount of food that you have between meals. Some people prefer a sweet snack, like a cookie or some chocolate; other people like a salty snack such as peanuts or potato chips. Similar to other meals or drinks, we use the verb have: the kids had their snack at 11:oo.
When people talk about healthy snacks, they usually mean some fruit or something better for you than what you'll find in the vending machine.
Snack can also be a verb, and it often goes with the preposition on: we shouldn't have snacked on those chips because now it's time for dinner!
Typical times for a snack are mid-morning or mid-afternoon, although we also have the expression "midnight snack" for those trips to the fridge to satisfy nighttime cravings...