"What are you up to?" is a casual way of asking "What are you doing?". It is a common greeting that can be used to start a conversation or check if someone is busy. The question can be interpreted in different ways, such as "What are you doing right now?" or "What have you been doing lately?". The response to the question depends on the context and can range from a simple answer like "Nothing much" to a more detailed explanation of what the person is doing. Overall, "What are you up to?" is a friendly and informal way of starting a conversation or checking in with someone.