Ain't is a contraction for am not, is not, are not, has not, and have not. It is not grammatically correct English. Native speakers use it, some people use it rather a lot but it is informal/colloquial English. In the wrong context it can give the impression of either laziness. or being poorly educated. It is for this reason that I am highly reluctant to recommend to non-native speakers that it is an appropriate word to use. But that seems like a bit of a double standard, that native speakers can use it and non-native speakers can't. But for a learner it can be very hard to judge the correct context, so with this in mind my advice would be not to use it until your English is of a high level. But definitely never use it in formal settings, and never use it in writing unless you want to specifically imitate colloquial speech.

am/is/are not, has/have not

It is the slang word for 'isn't'. It's quicker and quite a lazy option. It isn't polite and but not rude either.

am/is/are not, has/have not combined into one word to make it easier, but it is slang and normally not used in formal settings but it is okay in casual settings