meaning "what do you mean?"
it is a shortened version on "what are you going on about?" or "what are you talking about?"
usually used when someone is not making sense for extended periods of time or if you feel like you've missed something.
can be lengthened to show complete and utter confusion.
eg.
what the fuck are you on about?
what, exactly, are you on about?
what the fuck are you on about, exactly?
what the fucking
hell are you on about you fucking
cock? how's about you learn to
fucking speak english?
example 1.
knobhead: dude. i totally just pwned that
n00b. i asdfghjkl; the ask;nfw. fo shizzle my nizzle
dizzle.
awesome: what the fuck are you on about?
example 2.
knobhead: so, tomorrow, then?
awesome person: what?
knobhead: we can go and you can
meet them, yeah?
awesome person: what are you on about?