It could be either chips, candy, cookies OR food sold in the street.
Basically, what snack means is any food you eat that is not considered a real meal (breakfast, lunch or dinner). For example, you could say things like this:
"In between breakfast, lunch, and dinner, I eat snacks all day long."
"I ate so many snacks after lunch, I think I will skip dinner tonight."
"I need to eat fewer snacks, because I am gaining too much weight!"
"I like to eat small snacks whenever I drink tea."
You can also use the word "snack" as a verb. For example:
"I like to snack on apples in the afternoon."