It means “what are you doing right now?” When a someone calls you, for example, they might ask, “What are you up to?” and you might answer, “I’m just doing the dishes” or “I’m driving to work” etc. If you haven’t seen someone in a while, they might ask “what are you up to these days?” or something like that, and it has an implication of what you’re doing in your life lately. For example, the job you have, going to school, etc. I hope that helps!

It can mean "what are you occupying yourself with at the moment?" (which is the same as "what is keeping you busy at the moment?"), but it can also mean something like "what is the purpose of your suspicious behaviour?", depending on the context. For example, if a boy unexpectedly gives his girlfriend flowers and starts complimenting her, and she thinks that he is trying to distract her from something he has done or convince her to do something, she might say "what are you up to?" meaning "what is the purpose of this sneaky way you're acting?" Likewise if a teacher sees an older boy called Smith hanging a younger boy upside down by the ankles, the teacher might say, "what do you think you're up to, Smith?" or "what exactly are you up to, Smith?", to which the answer would doubtless by "we're only playing sir", as Smith swiftly puts the younger boy back the right way up. On the other hand if your friend rings you on the phone and asks "what are you up to these days?", that might mean "what are you doing to keep yourself entertained these days?", "what are you doing to keep yourself busy these days" or "what work are you doing at the moment?". "What are you up to right now?" might be something your friend would ask if they wanted to know if you are free right now to join them in some activity, or to speak to them for a while on the phone, or if they want to see if you are doing anything interesting which they could join in with.

Great answer from Rianne. I'm just adding that a common shortening of this phrase is, "What's up?"

It is another way of asking, what are you doing now? In English we also say... What have you been up to? This means what have you been doing since we last spoke/saw each other I've had many students ask me this in lessons :)