A fire-berry was a magical berry, described as being bright red in colour, which came from the valleys of the Narnian sun.
Each morning, as Caspian X observed on his voyage to the end of the world, white sunbirds came out of the sun to eat all the food that lay upon Aslan's_Table_(books). One of the white birds brought a fire-berry for the star Ramandu to eat, which he said that each fire-berry took away a little of his age. This would continue until the star was as young as an infant, at which point he would resume his dance in the sky.