The prayer leaf was a tradition among the people of the Gamma Forests. It consisted of a leaf-shaped piece of green cloth, embroidered with a child's name -- surname on one side, given name on the other. In "A Good Man Goes to War", Lorna Bucket explained to Amy Pond that this symbolised the belief that the child would always come home to her parents.
Lorna made a prayer leaf for Amy, embroidering it with Melody Pond's name, translated into the language of the Gamma Forests. The translation was inexact, however. When translated back by the TARDIS, "Pond" became "River" (because "the only water in the Forest is the river") & "Melody" became "Song". This gave rise to the name by which Melody Pond is better known: River Song.