The Otter's boy is missing. Portly, has been lost and Rat informs Mole of the missing child. They set out on the river at night to go look for the child of Mr. and Mrs. Otter. As dawn is breaking Mole begins to hear beautiful music from a pipe. Rat does not hear it at first, but as they go farther down river and the dawn becomes lighter Rat also hears the beautiful music. The two friends realize that they don't hear anything but the music.
"..the two animals moored their boat at the flowery edge of the island....the piping had stopped... There in the clearing stood the Friend and Helper. They could see the backward sweep of the curved horns, gleaming in the growing daylight. ...the long graceful hadn still holding the pan pipes. Last of all, nestling between His very hooves, sleeping soundly, Mole saw the little, round, pudgy, childish form of the baby otter."
The mythical creature had protected the baby and summoned the two friends with his musical pipe at dawn. This is the meaning of the title "The Piper At The Gates Of Dawn."
It basically means almost dawn- like dawn is just about to break. It describes the Greek demi-god Pan (god of mountain wilds, rustic music & other things) playing the pipes just before dawn. Is that ok?