Formal:
* நீ என்ன சொல்றீங்க? (Nee enna solringe?) - This is the most formal way to ask "What are you saying?"
* நீங்க என்ன சொல்றீங்கன்னு எனக்குப் புரியல. (Neenga enna solringeannu enakkup puriyala.) - This means "I don't understand what you are saying."
Informal:
* என்ன சொல்றீங்க? (Enna solringe?) - This is a more casual way to ask "What are you saying?"
* என்னப்பா சொல்றீங்க? (Ennaappa solringe?) - This is even more informal and uses a term of endearment, "appa" which means "father".
* என்ன சொல்றீங்கன்னு எனக்குப் புரியல. (Enna solringeannu enakkup puriyala.) - This is the informal version of the second formal option.
If you want to specifically ask what someone is talking *about*:
* நீ என்ன பத்தி பேசுறீங்க? (Nee enna pathi pesureenge?) - This literally translates to "What about are you talking?" and asks what the topic of conversation is.
Remember to use the appropriate level of formality depending on who you are talking to.