- "He knew which was the right tree at once, partly because it stood in the very centre and partly because the great silver apples with which it was loaded shone so and cast a light of their own down on the shadowy places where the sunlight did not reach. There, on a branch above his head, a wonderful bird was roosting. The tiniest slit of one eye was open. It was larger than an eagle, its breast saffron, its head crested with scarlet, and its tail purple."
- ―The Magician's Nephew.
The Tree of Youth (also, the Tree of Life) was the first, largest, and most spectacular silver apple tree in existence. It grew at the very centre of the Garden of Youth, and bore shining, silver apples that had wonderful, powerful magical properties, and gave off an ethereal, breathtaking, almost irresistible smell.
The tree was enclosed within the Garden, and roosting in its branches was a single phoenix (and the only one ever seen in Narnia). Though the apples were silvery and incredibly beautiful, their juice was darker than one would expect.
The first person to eat the Apples of Youth was Jadis who, dismissing the written warning that the fruits should only be plucked to help others, and not to be eaten for oneself, climbed into the Garden over the wall, and plucked a fruit for herself.
After she had greedily eaten the fruit, Jadis claimed that she felt such changes within her that she knew that she would never grow old or die. When Digory spotted her throwing away the core of the apple she had eaten, and saw how the dark juice stained her mouth horribly, he guessed - rightly - how she had entered the Garden, and thought he understood what the last line: For those who steal or those who climb my wall, shall find their heart's desire and find despair meant, for, despite the fact that Jadis looked "stronger and prouder than ever, and even, in a way, triumphant", her face was "deadly white, as white as salt".
Presumably ignorant of what she had doomed herself to, Jadis tried twice to tempt Digory into disobeying Aslan: first, by encouraging him to eat the fruit himself, telling him that it would make him alive and young forever. Second, telling him to give the fruit to his ill, dying mother instead, assuring him that it would cure her of her illness. Digory, very fortunately for him and his mother, was able to resist both temptations, and even angrily rebuffed Jadis, who retorted by calling him a fool to throw away his one and only chance of endless youth.
When Jadis began to feel the dark and cold inside her, she fled from the Western Wild, to the far north, to presumably begin creating her army. However, as Aslan said, it was actually Jadis, not Digory, who was a fool, given that the fruit would never work happily for any who pluck it at their own will, and that "length of days with an evil heart is only length of misery, and already she begins to know it" - Jadis' immortality meant that the misery that constantly plagued her because of her dark, evil heart would never end. Digory, by resisting the two devastating temptations, actually saved himself and his mother from terrible fates that would have definitely befell them if he had succumbed to either one of the temptations.
When Digory returned to Aslan with the Apple of Youth, Aslan told him to throw it on the bank of the Great River of Narnia, where it grew into the Tree of Protection that protected the Kingdom of Narnia from all enemies for 898 years.