Here's a fantasy book that tells you exactly what it's going to be about: you see, there's this wizard and he lives on Earthsea. The title may (or may not) be clear, but we can still break it down, word-by-word.
First, although Ged will become a super important wizard in the future, right now he's simply one of many – he's not The wizard, he's just A wizard.
Second, this novel tells about Ged and how he becomes a wizard.
Third, in some ways, this novel is about how Ged learns that he's part of the world (and the natural order). In that way, he's not just a wizard on Earthsea: he's a wizard Of Earthsea. He belongs to the world.
And fourth, Earthsea is where this story takes place and where Ged is a wizard. (Now, why is it called Earthsea? We're not sure, but how about this: because it's made up of earth – mostly islands – and lots of sea. And because "Sea-earth" sounds weird.)