A chum is a friend or a pal. You and your best chums might spend the weekend camping together.
The informal word chum is more common in Britain than the US, but you can still talk about your school chums or the new chum you made on a Caribbean cruise. Another kind of chum couldn't be more different from a buddy: it's chopped up fish parts used as bait on a fishing boat. This meaning comes from the Scottish chum, "food," while the first definition was Oxford University slang for a roommate or "chamber fellow."