Asks what another has his attention on. What is he thinking about or busy with? "Up" in this case is using a slang definition from around the 1930s meaning: occupied with, busy with.
This question is often used to start conversations with.
When I called my friend he didn't seem happy so I asked him "What is up?". He said he was having trouble with his girl.
An expression commonly mistaken with "what's the haps", What is you up to is the complete opposite. The expression simply means to wonder as to what activities another individual is partaking in at any given moment.
brotha:YO, what is you up to
hommie:just chillin with some peeps.
brotha: fly!