Give significance of the title "Piper at the gates of dawn"

Rat and Mole are searching for Otter's son late at night. They dravel down the river when rat seems transfixed and then begins to cry, saying that he heard a beautiful melody that made him extremely happy. A moment later, Mole hears the song, a tune coming from pan pipes. Entranced, they follow the music to a small island in a narrow stream. On the island, they see a faun, holding pan pipes and standing over a sleeping otter. (He is an allusion to the Greek demigod Pan.)