Appearance is the term used in The Lion, the Witch and the Wardrobe to describe the act of moving between the real world and the magical land of Narnia. It is not clearly explained how this happens, but it seems to involve a combination of magic and belief. The children first enter Narnia through a wardrobe in the Professor's house, but they are later able to appear there simply by wishing it.
Appearance is a significant concept in The Lion, the Witch and the Wardrobe because it allows the children to escape from the real world and enter a world of magic and adventure. It also emphasizes the importance of belief, as the children are only able to appear in Narnia if they truly believe in it.