The word "ain't" originated as a contraction for the words "am not", which started out as amn't and later evolved into ain't (probably because it's easier to pronounce). Over the years, the word "ain't" also replaced "are not" and "is not" and their respective contractions, but only in very informal settings and amongst the less-educated. It is not proper English, but you will hear it a lot in the southern parts of the U.S.

Basically means "isn't ", it never HAS to be used. You can use "is not" or "isnt". But if you want to be more casual, you can use aint