It simply means "到底“ in Chinese.^^ For example, 도대체 뭘하는거야? = 你到底在干什么呢? = What on earth are you doing?("on earth" is often found in British English^^) = What in the world are you doing? = What the hell are you doing? Also it can be used in a negative sentence. 도대체 무슨말인지 하나도 모르겠다. = 我真的不明白你到底说的什么。 = I don't understand what you are talking about. ^_______^

도대체 어떻게 안 거야? How did he know? 도대체 누가 알겠습니까? Who in the world knows? 그는 도대체 얼마나 마신거야? How much did he drink? generally it's not necessary in the sentence. you may think about 'hell'. What the hell was that all about? -> What was that all about? 도대체 그게 무슨 일입니까? -> 그게 무슨 일입니까? Can you understand?