Definition
To ask someone what they are on about is to inquire, often with a hint of annoyance, what they mean.
（常用于略带恼怒地询问）意指某人想表达什么。
I really don't know what you are on about.
我真的不知道你在说什么。
He kept complaining, but I couldn't understand what he was on about at all.
他一直在抱怨，但我完全不明白他在说什么。
What are you on about? What's your point?
你在说什么？你的重点是什么？