This is a slang expression and it means "you don't have to" expressing obligation. IT could mean "there is no need" in this situation.

In addition to what others have already said, ain't is generally only used as a slang term in the southern United States. Nobody here in Canada uses it (not even those with low levels of education), and I'm not aware of that term being used in any other English speaking country. It's often heard in films though, which is probably why it's a well known term.

It is slang and it is also Very Bad English in some cases in Urban cities in the U.S. (east coast). I've heard expressions such as "I aint doing it" meaning I am not going to do it. Or "She aint got no money" meaning She doesn't have any money. Bottom line, don't become too familiar with "aint".

I understand. Most native speakers of an acceptable level of education do not use "ain't" except in jest. You should not be talking to people who talk like that. I shall be criticised for being a snob for saying this, but that is my honest observation.

*a native speaker