Hello bro, I guess "Ain't" denies the next word, for example,  from Bon Jovi's song

"I ain't gonna be, just a face in the crownd" means "I wont be, just a face in the crownd"

I hope it help you.

Just remember the phrase my grandpa would say: "Ain't ain't a word and I ain't gonna use it!" 

But seriously,

Ain't, depending on the context means "am not" or "is not" or some other form of "to be + not".

Hope this helps.