Its slang. Originating from a TV beer commerical I think. It just means Whats up? Whats going on? Whats happening? That sort of thing.

It is just a verbal shortening of "what is up?" but it is spoken rhetorically, and with great emphasis. It really is just a greeting, no response required (except maybe "WAZZZZZZZZZZZZZZZUUUUUUUUUUUUUUUUPPPPPPPPPPP"

It's like "What's up?" or something like... "How are you?"