아니다 is an adjective used for denying something. A은/는 B이/가 아니다 means "A is not B" 그가 한 말은 사실이 아니다 What he said is not true. 그날 당신이 본 사람은 제가 아니고 제 여동생입니다 I wasn't the one you saw that day. It was my little sister.

It is a Busan dialect of 아니야, which is actually No.