All of the books in the Narnia series have strong moral lessons, and in this one the main character, Digory (the professors' nephew) grows from a self-centered boy into a young man with principles. In the beginning of the novel, Digory is a miserable, whiny kid that is feeling sorry for himself because he is in London with his uncle and sickly mother. He likes to do risky things and does not think about the consequences. As a result, he and his friend, Polly, get into a lot of trouble.
However, as the novel progresses, Digory changes. He starts to think more about his actions. He matures. He learns to put people ahead of his own selfish wants. When his uncle Andrew comes under the spell of Jadis, the evil witch, Digory is the one who decides to bring her into Narnia. In doing this, he releases sin into that world (similar to the Adam and Eve story in the Bible). Luckily, though, Narnia is ruled by Aslan, whom Jadis cannot defeat. Aslan gives Digory a chance to redeem himself and sends him to retrieve a magic apple. Digory resists eating the apple, even though he wants to eat it very much and take one for his mother as well. In the past, he would not have hesitated to eat it. He now realizes that his mother would not approve of him stealing something, even if it will heal her.
In making the choice not to eat the apple, Digory illustrates that he has learned one of life's most important lessons: sometimes doing the right thing is hard and requires extreme sacrifice. Since Digory makes the right decision, Aslan rewards him and gives him the apple. Digory's mother is cured, and Digory plants the apple which grows into a tree, the wood of which he uses to build the famous WARDROBE that begins the other stories.
Lewis' message, I believe, reflects the Biblical truth that just because something is difficult does not mean we should not do it, if it is the right thing to do.