You ask "What are you (doing)?" to find out about someone's planned (future) actions. Asking in this way makes it sound like you think the listener already has a specific plan.

For example:

What are you doing tonight?

You can also ask other questions, like "Where are you...?", "How are you...?", "When are you...?", and so on.

A: I'm going out for a little while.
B: Where are you going?

In this example, Person B thinks that Person A already knows where they're going to go so Person B asks "Where are you..."