Hello, Madeleine Mamaradlo. There could be a lot of ways to say 'what do you mean' in Korean but normaly you can say 무슨 뜻이에요? museun ddeushieyo? (also you can use it as the meaning of 'what does (he/she/sth) mean?') 무슨 말씀이세요? museun malsseumiseyo? or if you want to make it familiar or rude, 무슨 뜻이야? museun ddeushiya? 무슨 말이야? museun mariya? are also okay.