Here it is replacing the word 'is' with 'isn't' which makes it a highly grammatically incorrect sentence because of the double negative. It is American slang , don't use it, just understand it.

Basically, 'Ain't' can be used to every sentence that you can deny with a 'not' in nowdays english. Aren't, haven't, isn't, won't, wasn't, weren't, etc. So, everytime you see an 'ain't', think about it as a deny to whatever is next in the sentence, and you get if it is past or future by the context. I hope this helps you!

In that sentence, ain't means "there isn't"

ain't = isn't Also slang for: aren't; am not; haven't; hasn't.