The fictional land visited through the wardrobe in C.S. Lewis’s Narnia series is Narnia. In The Lion, the Witch and the Wardrobe, Lucy Pevensie discovers a passage from a wardrobe into this other world.
Narnia is the setting of all seven books in the Chronicles of Narnia, although the stories also visit neighboring lands and other locations. The world includes talking animals, mythical creatures, human kingdoms, and the great lion Aslan. The wardrobe serves as the first major doorway between the ordinary world and Narnia.
Narnia is not the same setting as Middle-earth, the world of J.R.R. Tolkien’s legendarium, despite the authors’ friendship and shared interest in myth. Nor is it Earthsea, the archipelago created by Ursula K. Le Guin. Those names belong to separate fantasy series.