She is theantagonist in the story and represents an ever-present threat to the children, as indeed she does to the lion Aslan, who is an allegory of Christ. Though Jadis is defeated by Digory's refusal to give in to temptation she has not been completely vanquished. This is Lewis's way of saying that the struggle between good and evil goes on, and that the faithful must always be on their guard against the dangerous lure of the devil and all his temptations. That Jadis still remains a threat also foreshadows her reappearance in The Lion, The Witch, and The Wardrobe.

There are a lot of themes in this book.  Each reader has to think for him/herself about which are the main ones.  Here are a few themes:

• Maturation -- Digory matures a great deal over the course of the book.  He starts out impulsive and selfish, but grows as the book goes on.
• Difference between what's good and what's practical -- Jadis and Uncle Andrew only want what is useful and do not understand what is good.
• Creation and destruction -- this is especially seen in the destruction of Charn and the creation of Narnai.
• Forgiveness -- Digory is forgiven even though he harms Narnia by bringing Jadis there.

The Magician’s Nephew by C. S. Lewis is a story of adventure and maturity set against backdrop of the creation of the fictional land of Narnia. The main themes in this novel are good vs. evil and the concepts of growth and maturity and taking on responsibility.

The first theme is very clear—Diggory’s uncle is a selfish and vain magician who accidentally brings a powerful and evil witch first into our world, and then into the virgin Narnia. Diggory and Polly have to follow Aslan’s instructions to prevent her from destroying that world and keep her from causing more damage.

A second theme is that of responsibility. Diggory is a young, playful child who spends most of his days exploring his attic prior to his arrival in Narnia. He is shy and cautious in general, never having accomplished much, and he shies away from worrisome situations, such as his ailing mother. At Aslan’s bidding, however, he takes a dangerous journey to this extreme North of Narnia and saves the day while providing for for his sickly mother in the process. He matures and comes to accept his responsibility through the events of the novel.

While The Magician's Nephew is the sixth book published as part of the Narnia Chronicles by C. S. Lewis, it is actually the first in chronology for the series--a prequel, if you will, to The Lion, the Witch, and the Wardrobe.  From your question, I can't tell whether you've already read the book or if you're looking for some things to look for as you read.  I'll try not to give away too many of the plot elements in case you haven't read, but either way, this is a book full of interesting and symbolic issues to think about and study. 

The Chronicles are all allegorical in nature, so the primary element to look for in this novel is the allegory--the characters and places which represent things greater than themselves.  Consider the following:

• Aslan sings and creates a world--from nothing.
• Jadis, the ice queen, is an enemy to Aslan.
• Aslan and Jadis are locked in a battle between good and evil.
• Gardens and an apple and temptation are all part of this story.

One other important thing to examine in this novel, especially if you're reading the entire Chronicles, is all the "beginnings" in this novel which carry over into the rest of the series.  And if you're looking for something more in-depth to study and/or research, perhaps you could examine why this novel is out of chronological order in the series.