Idunn, the goddess of spring and rejuvenation, was also the keeper of the Apples of Immortality. A vital part of keeping the Norse gods young and beautiful and, well, alive.
Her story is one of many involving Loki causing mischief, where he needs to get himself out of trouble and gets Idunn kidnapped to do so.
Without Idunn and her golden Apples of Immortality, the gods started to age quickly. They knew Loki was behind it, because he was always behind trouble. They forced him to rescue her using Freyja’s cloak of falcon feathers; transforming him into a bird to swoop in and save Idunn from the giant. All so they could have their Golden Apples of Immortality.