"What are you on about?" is a British idiom that means "what are you talking about?" or "what do you mean?". It is often used when someone is not making sense or talking about something at length. The phrase can also be used to ask what someone is doing or what their plans are. In some parts of London, "what are you on" is slang for saying "what are you up to". However, it is important to note that the phrase can also be used in a rude or accusatory way, implying that someone is high on drugs.