It means "isn't" or "aren't" but it's definitely substandard/slang -- You might come across it in song lyrics or in literature where the speaker is uneducated, but don't use it in conversation or writing if you want to sound reasonably intelligent.
There are some fixed phrases that use "ain't" like "Ain't love grand?" or "Say it ain't so!"
However, as khoff has said, don't use this word, or at least, don't use it unless you have a very good feel for whether it will sound colloquial and appropriate or whether you will sound ignorant.
I actually do use it, but always for a deliberate effect - not simply as part of my regul