This novel in the Narnia series takes place during World War II. Lucy, Peter, Edmund and Susan enter the land of Narnia through the wardrobe. While in Narnia, Edmund becomes bewitched by the White Witch, who has turned all Narnia into deadly winter. Through many adventures, the children try to free Edmund because the White Witch is going to kill him for being a traitor to her. Aslan offers to ransom himself for Edmund. According to the deep magic of Narnia, the White Witch reminds Aslan that she has control over all traitors, but she accepts Aslan's offer to die in Edmund's place.
Aslan is kiilled by the White Witch but comes back to life through "deeper magic." When a being who is blameless willingly dies on behalf of a guilty person, the dead person may return to life.
This story is an allegory for the death of Christ (and His subsequent rising from the dead) to save sinners. Edmund is the sinner and Aslan is the Christ figure. Aslan is without sin, just as Christ was without sin, and Christ died on the cross so that sinners could be saved and enter heaven because "the wages of sin is death."
C.S. Lewis, the author, was a great Christian writer, in addition to being a famous professor. You can read all about it here on enotes.
The solution is that Aslan paid the price for Edmund's sin, just as the solution for the sins of mankind is Christ's death on the cross, in the Christian faith.