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

• Informal: museun ddeut-iya? (무슨 뜻이아?), museun ddeut-ini? (무슨 뜻이니?)
• Formal: museun ddeut-iyeyo? (무슨 뜻이에요?), museun ddeut-ipnigga? (무슨 뜻입니까?)
• Very Formal: museun ddeut-ipsipnigga? (무슨 뜻이십니까?)

- what (objective): museun (무슨), 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) - omitted here
- to mean: ddeut-hada (뜻하다), euimi-hada (의미하다)

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