It means isn't, or is not. It is slang English, common in America and parts of England and should not be used in formal writing. It is used in place of isn't.

"ain't" is a slang form of isn't, aren't, am not. It can replace any of those in a sentence, but is very informal and therefore should never be used in formal writing or speech. Example: I ain't got nothing better to do. = I have nothing better to do. She ain't coming. = She is not coming. They ain't stupid. = They are not stupid.

ofcourse, thank you very much :)

The two answers below are basically correct, but I would suggest not using it even in regular conversation. People tend to associate it with a poor education. Know it so if others use it you know what they are saying but there is no need to ever use it yourself.