It's slang for "has not" or "is not". It's common in England, USA and Australia but not every English-speaking country. Many people don't say it - I don't. I don't recommend you consider using it unless you move to one of these three countries, live in a community where people commonly use it, and become very fluent in slang English.

Yes it's mainly used in American southeast as a negative as in " is not " or " has not" or " will not" vivo en Kentucky donde "ain't" es muy common. Aqui lo useó a un negativo por los modos distintos. Lo siento. mi espanol no es Bueno. Pero espero que estas ayuda a usted.