"What are you up to?" is a common way to ask someone what they are currently doing in Engpsh.
This phrase is pkely a shortened version of the older expression "What are you up to these days?" which was used to ask about someone's current activities.
Here are some example sentences and their translations:
"What are you up to today?" (今天你在做什么?)
"I'm just hanging out at home, what about you?" (我只是待在家里，你呢?)
"Not much, just catching up on some work. What are you up to?" (没什么，只是补一些工作。你在做什么?)
Note: "up to" in this context means "involved in" or "occupied with".