What are you on about is correct?

Asking: "What are you on about?" is an informal everyday way of asking what the speaker means (almost asking if the speaker really knows anything about the subject which he claims to be talking about). But note that it is an informal expression and would not be seen in formal writing.

In Britain also (not only in USA), some people ask: "What are you going on about?"

"What are you on about?" is an expression that is mostly limited to British English, and I
believe it's perfectly normal in the UK.

In the US, we'd probably say "What are you going on about?"

PS: A bit of punctuation would make your question easier to understand.

I'm sorry. Your question makes no sense.

Not sure what you are asking, but I assume you mean

Is 'What are you on about"? a correct sentence?

I would write "What are you going on about" instead, that's better