Its southern slang. Sometimes it means "do not" or "am not"... 1. You ain't got an umbrella? 2. I ain't going outside in that rain. You don't really need that word because its bad grammar.

It is technically "bad grammar" but if you are watching a lot of English movies and TV shows, you will hear "ain't" all the time. As everybody says, it is probably not a good idea to use it, but in general it is a shortened form of "are not," "am not" and "is not"-- it works for every conjugation of English: I ain't, you ain't, he ain't, we ain't, y'all (you, plural: only used in the South and in certain classes, but you might hear it) ain't, they ain't. I actually do say "ain't" sometimes-- it's a good word to know when you hear it, but it does not sound nice unless you know exactly when to use it.

Yes, you've guessed correctly: "You didn't have to..." "Ain't" is a non-standard negative word, but using it will mark you as someone from a specific social class or region. Really, as a learner you'll have almost no reason to use it, unless your native English-speaking friends actually speak like that.