Based on the level of respect or formality, you can say "What are you talking about?" in Korean in 3 ways:

• Informal: neo-neun mwol mal-hago itseo? (너는 뭘 말하고 있어?), neo-neun mwol mal-hago itneungeoni? (너는 뭘 말하고 있는거니?), neo museun sori-ya? (너 무슨 소리야?)
• Formal: dangsin-eun mueot-eul mal-hago itseupnigga? (당신은 무엇을 말하고 있습니까?), dangsin-eun museun sori haneun geoyeoyo? (당신 무슨 소리 하는거예요)
• Very Formal: dansin-ggeseo-neun mwol malsseum-hago gyesipnigga? (당신께서는 뭘 말씀하고 계십니까?), dansin-ggeseo-neun museun sori hasipnigga? (당신께서는 무슨 소리 하십니까?)

- what (objective): mueot-eul (무엇을), mwol (뭘), mwo (뭐)
- you (subjective): neo-neun (너는), dangsin-eun (당신은), dansin-ggeseo-neun (당신께서는), ne-ga (네가), ni-ga (니가)
- talk about [something]: [eotteon-geot]-e daehae mal-hada ([누구에] 대해 말하다)

Please refer to How to pronounce Korean Alphabet for the English pronunciation in this answer.