It's not exactly a snack, but not quite an appetizer either. 小吃 can be eaten as a snack between meals, or you could make a meal out of around 3-6 different types of 小吃. It's like small plate dining, but not as upscale. Think the food you get at stalls in Asian night markets, or typical fairground/carnival food, for a western equivalent; except compared to carnival food, 小吃 has a lot more variety and not just sugary treats for kids. You can paste the word 小吃 into google and look at the images, to give you an idea.