It's an American southern slang or improper grammatical contraction, probably of "are not", but it can be used like "is not" as well. It is very informal and some people view its use really poorly. I used to say it a lot because I grew up in the south, and when I moved to other places I got made fun of. Because it is associated with the south it is associated with rural ignorance here. I'm not saying this is a good thing, it's just the way it is. You use it like this "There ain't any soda in the fridge". "He ain't ever gonna get here".

It's a contraction of am and not 'ain't', I ain't doing that -> I am not doing that. However it does get used incorrectly as the two answers before this point out, it get's used in the place of aren't and isn't when it shouldn't be.

The word often used to make fun of those who speak English incorrectly. It is important to know the word so you know to avoid it. Song lyrics often contain words that are substandard English. If you like the music then songs are a good way to learn new words. Just take care, not all words in American songs are good words.