It means nostalgia for a place you've never been. Or people you've never met. Or maybe people or places that never existed at all. I mean, they're never as real as they are in your mind. Right?
The fundamental mistake I had always made—and that she had, in fairness, always led me to make—was this: Margo was not a miracle. She was not an adventure. She was not a fine and precious thing. She was a girl.