Ain't is a slang way of speaking and we shouldn't really use it in a formal situation. It generally means negative when we use with auxiliary verbs to be and to have
For example, we can say:
- I ain't going which means I am not going
- I ain't got any money - I haven't any money
- Ain't you heard the news? - Have you heard the news?
I think it is more British English than American English