Aslan gives Digory a piece of fruit to take with him to Earth. Aslan gave him this fruit for his bravery and for not giving into Jadis's temptations. Digory brings the fruit to Earth and gives to his sick mother so that she can become healthy again. After she eats the fruit, Digory takes its core and plants it in his yard. A tree grows from the core just as it had back in Narnia. The wood from this tree later is used to build the wardrobe in which Peter, Lucy, Edmund, and Susan use as a passage way to Narnia.
I hope this helped! :)
