What was Digory's quest?

From the text:

"You see well," said the Lion. "Now the land of Narnia ends where the waterfall comes down, and once you have reached the top of the cliffs you will be out of Narnia and into the Western Wild. You must journey through those mountains till you find a green valley with a blue lake in it, walled round by mountains of ice. At the end of the lake there is a steep, green hill. On the top of that hill there is a garden. In the centre of that garden is a tree. Pluck an apple from that tree and bring it back to me."