As titles go, this one follows the same formula as that classic Western, The Good, the Bad, and the Ugly.
We've got the good: the Lion, Aslan, who stands for truth, beauty, compassion, and everything that's right in the world.
We've got the bad: the Witch, who stands for evil, lies, treachery, and everlasting winter.
And we have the ugly: that wardrobe is more heinous than anything you can find at IKEA. Burn it, please.
Okay, we lied. The wardrobe is a classy piece of hardwood furniture...and it's also the gateway to the book's magical world, in which good and evil are clear-cut and myth mixes seamlessly with religion. But the wardrobe can also stand in for the real world of 1940's England, which existed in the moral gray area of reality.
The interesting thing about this title is that, unlike The Good, the Bad, and the Ugly, the meaning of the different terms isn't obvious just from hearing or reading the title.
In order to know that the Lion is a personification (or would that be a lion-ification?) of all that's good in the world, or that the Witch is his enemy, or that the wardrobe is a gateway to another world, you have to read the book.
So the title does several things: it sets up the great battle between Good and Evil that structures the story, but it does so in an undercover way that requires decoding. This is the same pattern that we'll see throughout the book: the values of the world of Narnia are extremely straightforward, but they're under a veneer of mythology and fantasy.