Someone asked me something without including the subject. I don't know what they were referring to, so how would you go about asking for the subject of what they are talking about, or just generally asking what they are talking about?
Japanese's particle system comes in handy here.
何【なに】が（ですか）？ (for asking the subject)
何【なに】を（ですか）？ (for asking the object)
Exchanges like this can be difficult to translate into English...
Or for instance:
You can use "何の話？", "何のこと？", "何の話ですか？", or "何のことですか？"
- A:「すごかったよ！」 B: 「え？ 何の話？」 (casual)
- A:「明日までにやってください。」 B: 「すみません、何のことですか？」 (politer)
- (jumping into a conversation)「ねえねえ、何の話？」
何 here can be read as either なん or なに, but the latter sound politer.
Note that using "言う" in this situation can sound accusatory.
- 何を言っているの？ ≒Are you kidding? What are you talking about?
- 何を言っているのか分かりません。 ≒What nonsense you are talking!
This really depends on situation and the people you are talking to.
Usually a semi-polite
何についての話ですか？ (About/Referring to what are you talking about) would work for most situations.
There are more formal and less formal ways of saying it though depending on what social position the other person is though.
Less formal would be
More would be
This is in no way an exhaustive list and the context of the situation generally denotes the way you are expected to ask.