It tells
of probbably of childhood lovers. But the girl move away and after some years they meet each other again but the girl the guy once love change into a different person; its like he didn't knew her anymore, he asks in the song what happened to the girl he once love over the passed years.
It shows how a person can change over the years, and how time can change everything. We could lose the people we love and knew after being apart from them for a long time.
Speaking from experience Xp.