I think this beautiful song has many meanings. In the movie, I think it refers to Aslan, who is always the Savior of the movie. If you read about C. S. Lewis, he was first an atheist in his earlier years. He later became a very devout Christian. The Narnia books have many parallels with Christ. Aslan is Christ-like, in that he is a Savior. Lucy believes in him the most and is very faithful. Aslan asks her why she listened to the others instead of following her beliefs in "Prince Caspian". This song can apply to Aslan/faith in God. Listen to the song and watch the movie with this in mind and you will see how C. S. Lewis applies his beliefs to the stories. I think the song talks about both Aslan/Christ coming back when you call Him, as well as you going back/following your belief in God.