"It don't mean nothing" = "It means nothing."
Double negatives are interesting. They are bad English. They sound uneducated. OR, they sound like someone speaking under intense emotion and forgetting to use good English.
Double negatives are common in spoken English. They might be used in a song to sound more authentic, more "real." Everybody understands double negatives. Using a double negative makes the meaning MORE negative. More "no-words" = stronger sense of "no."
Sometimes people speak that way to sound tough--because they do NOT want to sound educated.
These are examples of bad English. Understand them, don't use them.
"You ain't heard nothing yet"--Al Jolson Means: you will hear a lot more.
"I was scared, but a man says: 'He don't mean nothing; he's always a-carryin' on like that when he's drunk.'"--Mark Twain, "Huckleberry Finn" means: "Don't be scared, he doesn't mean it, he's drunk."
"'But, Lord! It don’t MEAN nothing!' said the old flower-seller."--H. G. Wells, "Boon"