This novel, as well as the others in the series The Chronicles of Narnia, is a Christian allegory. One of the main themes is the triumph of good over evil. The White Witch (Queen Jadis) represents evil and Aslan represents good. Aslan is a Christ figure. He is willing to lay down his life for others, just as Christ was willing to do.
Another theme is doing what is right. Digory knows the magic apple can cure his mother, but he also knows that if he takes it, it would be stealing, and his mother does not approve of stealing. So he does what is right, over what he wants to do, and is rewarded for his choice because Aslan gives him an apple anyway to cure his mother.
Finally, when one is willing to lay down his life for another, that is the theme of love, and love is an important part of this novel. Digory shows love to his mother, Aslan shows the greatest love of all.
Aslan himself is a symbol for Christ. The apples are a symbol of choices, kind of like the tree in the Garden of Eden. The apples are not evil in and of themselves, but when one is told to stay away from them by God himself, then taking one IS a sin. Adam and Eve did not obey this command, but Digory does.
Other symbols are the pools (portals from which to go from one world to another). The pools are like houses. Each one is different and sometimes what one finds is good, sometimes what one finds is evil. The magic rings are symbols of evil. In the wrong hands, they can lead to all sorts of mischief. That is why Digory buries them at the end. Jadis is a symbol of evil. The bell is a symbol of temptation.
What are some other symbols that YOU have found?