Most often, it’s just a casual question to find out how someone is occupied at the moment.

But, sometimes it is asked by someone who is suspicious of what you might be doing, thinking you might be “up to no good” (doing something you shouldn’t be doing).

It basically means “what are you doing”.

If someone believes you’re sneaking around and doing something wrong, they may mean “what are you trying to get away with doing wrong?”

If they are simply curious, they mean “what are you doing – or what have you been doing just now”

If they are concerned about your health, they may mean “what are you physically able to do?”

Mostly though, it is just another way of saying “what’s happening?” or “what’s going on?” or “what’s shaking?”