The following is a translation of the story of Iðun from the Skáldskaparmál (The Poesy of the Skalds) by Snorri Sturluson as part of his Prose Edda.
He began the story at the point where three of the Æsir, Odin and Loki and Hœnir, departed from home and were wandering over mountains and wastes, and food was hard to find. But when they came down into a certain dale, they saw a herd of oxen, took one ox, and set about cooking it. Now when they thought that it must be cooked, they broke up the fire, and it was not cooked. After a while had passed, they having scattered the fire a second time, and it was not cooked, they took counsel together, asking each other what it might mean. Then they heard a voice speaking in the oak up above them, declaring that he who sat there confessed he had caused the lack of virtue in the fire. They looked thither, and there sat an eagle; and it was no small one. Then the eagle said: “If ye are willing to give me my fill of the ox, then it will cook in the fire.” They assented to this. Then he let himself float down from the tree and alighted by the fire, and forthwith at the very first took unto himself the two hams of the ox, and both shoulders. Then Loki was angered, snatched up a great pole, brandished it with all his strength, and drove it at the eagle’s body. The eagle plunged violently at the blow and flew up, so that the pole was fast to the eagle’s back, and Loki’s hands to the other end of the pole. The eagle flew at such a height that Loki’s feet down below knocked against stones and rock-heaps and trees, and he thought his arms would be torn from his shoulders. He cried aloud, entreating the eagle urgently for peace; but the eagle declared that Loki should never be loosed, unless he would give him his oath to induce Idunn to come out of Ásgard with her apples. Loki assented, and being straightway loosed, went to his companions; nor for that time are any more things reported concerning their journey, until they had come home.
But at the appointed time Loki lured Idunn out of Ásgard into a certain wood, saying that he had found such apples as would seem to her of great virtue, and prayed that she would have her apples with her and compare them with these. Then Thjazi the giant came there in his eagle’s plumage and took Idunn and flew away with her, off into Thrymheimr to his abode.
But the Æsir became straitened at the disappearance of Idunn, and speedily they became hoary and old. Then those, Æsir took counsel together, and each asked the other what had last been known of Idunn; and the last that had been seen was that she had gone out of Ásgard with Loki. Thereupon Loki was seized and brought to the Thing, and was threatened with death, or tortures; when he had become well frightened, he declared that he would seek after Idunn in Jötunheim, if Freyja would lend him the hawk’s plumage which she possessed. And when he got the hawk’s plumage, he flew north into Jötunheim, and came on a certain day to the home of Thjazi the giant. Thjazi had rowed out to sea, but Idunn was at home alone: Loki turned her into the shape of a nut and grasped her in his claws and flew his utmost.
Now when Thjazi came home and missed Idunn, he took his eagle’s plumage and flew after Loki, making a mighty rush of sound with his wings in his flight. But when the Æsir saw how the hawk flew with the nut, and where the eagle was flying, they went out below Ásgard and bore burdens of plane-shavings thither. As soon as the hawk flew into the citadel, he swooped down close by the castle-wall; then the Æsir struck fire to the plane-shavings. But the eagle could not stop himself when he missed the hawk: the feathers of the eagle caught fire, and straightway his flight ceased. Then the Æsir were near at hand and slew Thjazi the giant within the Gate of the Æsir, and that slaying is exceeding famous.
When we work with Iðun, we seek to keep young, both mentally and physically. She is renewal, offering Her apples to the Gods to ensure that They continue to live, ever youthful and full of vibrancy. Without Her, the Gods would grow old, and die. Her role is paramount to the Gods’ survival, and while we humans do not benefit from Her apples’ abilities to keep us forever young and immortal, we benefit from working to remain youthful at heart.