What Does "What's Going On" Means?

It depends upon who is asking the question and what context the question is being asked in.

IE: Same as "Whats up ?", "Whats new ?', or "How are you ?"
Basically a greeting.

When asked this by a parent or authority it probably means "What are you up to ?, or "What are you into now that we should know about ?" or at the scene of some accident or other event, when asked by a new arrival at the scene. "What is taking place here ?"
These are just a few examples of what the question could mean.

It means what's happening or what are you doing.  Another version
of this (from when I was younger) is "What's the happs?"  Hope
this helps. :)