It means "what are you talking about?" (what are you on about = what are you going on about)

"to be on about" something is an idiom (commonly used by the British) which means to be talking about something at length. So "what are you on about?" means "what are you talking about?".

What are you on? It could be "Are you high?" Are you on drugs?" or in a more positive light: "What are you about to do?" What are you doing now"? It all varies on the persons intention.

do you mean to ask: what are you on? i.e. Are you high? , What type of drugs are you on? :P