Other forms: real worlds
The real world is reality, rather than an idea, fantasy, or dream. If your dad tells you, "You need to live in the real world!" he probably means that you should be more practical and face your responsibilities.
People who "live in the real world" generally have successful careers or serious plans for the future. If you refuse to live in the real world, you might be impossibly romantic or just impractical. People who are absolutely certain they'll win the lottery don't live in the real world. But then again, your friends might accuse you of not living in the real world when you decide to be a movie star — and who knows, you might get a lucky break!