Meaning:
I don’t understand what he is trying to say. It all sounds mad to me

Synonyms:
I am not sure what he/she is on about
What are you on about?

Sentences：
Couldn’t understand what he was on about.

Conversation 1
A: How is your new job?
B: Oh the job is ok, but I have a bit of a problem with the manager.
A: Really, what’s the problem?
B: He has a really strong accent so I am not sure what he is on about half of the time.
A: Oh that’s not good, you should ask him to speak a bit slower.
B: I thought about that, but I don’t want him to think that my English isn’t very good.

Conversation 2

A: Oh god no…
B: Huh?
A: How could he?
B: What?
A: And with her…
B: Who?
A: I’m going to kill him
B: Wait, what are you on about?
A: My boyfriend
B: What? Slow down.
A: I found my boyfriend kissing the girl from the club with the short skirt.
B: Ah, I see…

Conversation 3 

There you go, it’s a bit tricky to fit in to conversations and make people understand the conversation, and otherwise it would be
A: jhfbkvfnvknmmph
B: Huh?
A: ljegfouhefojk kufhlem clhf jewf klju4 oirkn
B: What are you on about?

(callan book stage 8, 721p)