"What are you up to?" is an informal way of asking someone what they are doing or what their plans are. It is often used as a conversation starter or a way to catch up with someone. The phrase "up to" in this context means "doing" or "involved in". When someone asks "What are you up to?", they are usually looking for a brief update on what the person has been doing or what they plan to do next. The response can be anything from a simple answer like "just hanging out at home" to a more detailed explanation of what the person has been up to lately.