Denotes whether a person is someone you know or not. If they are not real to you, you don't care about them and can joke about them without feeling guilty.
You might say, "He's not real to me," about someone you see at work but have never talked to.
People you swear at while driving in your car are "Not real to you." However, if they waive and you realize they are your best friend's Mom, they become real to you and you hope they didn't notice when you flipped them the bird.