Idunn is the goddess that carried the apples of eternal youth. Not surprisingly, her domains are therefore considered to be the goddess of both apples and youth. She is the wife of Bragi, the son of Odin and Gunnlöd. Her own origins have not been recorded.
Given her domains, she has an important role to play in Norse mythology. She is the keeper of the apples of eternal youth, which are used by the gods to stay young. If she would somehow disappear from the scene, that would mean that the gods would lose their strength by growing old and brittle.
This is exactly what happens when she is kidnapped by the goddess Thiassi, after she has been lured into a trap by Loki. Luckily enough, Loki is able to return her to Asgard in time to save all of the gods.