What does that mean when a girl wave at you?

It can mean a couple of things. It can mean they like you, at least as a friend. You should wave back and talk to them, but it can also mean they don't like you. For example, you look at them because they're pretty and then they wave at you to draw attention and get you to stop staring at them.

Well, it depends. If somebody waves at you, they might be saying hi, or if somebody waves at you in a flirty way, they could like you.