Can anyone please tell me what this error means?

It says - re-check (user) ID

아이디 중복 확인을 해주세요.

Breaking this down word by word...

아이디: transliteration of the English acronym "ID"

중복: duplicate or repeat

확인하다: to confirm

해주세요: please do

This is something you see at most Korean websites that require that you register. During the registration process, next to the box where you type in your desired user ID, there will usually be a button similar to the one shown in the original post. It's simply asking you to confirm whether or not the ID you've chosen is already in use. Of course, if it is, it will let you know that it is, and you can then choose another ID.

Actually the word to "reconfirm" is 재확인. "재" (再) is a prefix used often in the Korean language indicating that something is being done again. For example:

재혼 - to remarry
재심사 to re-examine

it means that ID is dupulicated.. in other words, ID you fill out is already registrated ID by another user.

아이디= ID, 중복 = DUPULICATION

if you have another quetion , feel free to mail to me..
mail: go4lovee@daum.net