It's the formal way to say "you are going" in French. It's used when addressing someone you don't know well, someone older than you, or someone in a position of authority.
Here are some examples:
* "Vous allez au cinéma?" (Are you going to the cinema?)
* "Vous allez bien?" (Are you doing well?)
* "Vous allez voir un film ce soir?" (Are you going to see a movie tonight?)