here, it means "It is nothing" " ain't " is more of a slang way of saying "it is not" but people who use "ain't" are commonly considered uneducated. especially shown in that sentence because literally "ain't nothing" says "it is not nothing" and it would be a double negative, which is frowned upon in the english language. but it's known that when one says "ain't nothin", they do mean "it is nothing". Listening to the Backstreet Boys, huh?