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?

Referring to the heavy unfashionable winter jacket and patterned mittens Sanders wore to the inauguration of Joe Biden.

Yorkshire slang. Another way if asking someone what they are talking about.

guys stop being mean to ben and just fuck off ben im with you