Historically speaking... "ain't" used to be a perfectly acceptable contraction for the first person singular. I am = I'm .... I am not = I ain't You are = you're ... You are not = you're not etc. But even though historically it stood for "I am not," it is now applied to any subject, really! You ain't wearing that shirt tonight! It ain't raining now. We ain't paying for it. They ain't worried. It is considered poor grammar to use the word, even as a contraction for its original use, "I am not". You can certainly say it or write it in casual conversation, but never say or write it in a professional environment. It is also more commonly heard in certain regions of the US, like the south. ...That's probably way more info than you needed, but I hope it helps!!

Ain't is a contraction for am not, is not, are not, have not, has not. Most grammar teachers will say NEVER use it. The fact is, many people do use it, mostly in informal contexts. I would not use it in formal situations. You will find it in books, such by Charles Dickens and Mark Twain, to represent certain native dialects. It is also used in several fixed expressions or idioms: "If it ain't broke, don't fix it." (If something is not broken, don't try to fix it. Or: If something is working well, don't try to change/adjust/fix it.) "You ain't seen nothing yet." (You haven't seen anything yet, compared to what I willl show you now/later.)

"Ain't" is an American phrase which means "is not".

It can also mean "am not" e.g. I ain't going

It's also slang and not proper English, so don't use it in an English composition or grammar class.