Only in silence the word,
only in dark the light,
only in dying life:
bright the hawk's flight
on the empty sky
- The Creation of Ea
First things first: if you try to find The Creation of Ea in your library, you're not going to have much luck, unless the librarians are awesome and magical. (Which, honestly, could happen.) Why? Because The Creation of Ea only exists on Earthsea – it's one of the songs that Ged has to learn when he goes to Roke school (3.55). It's one of those history songs that tell about the past of Earthsea – in this case, about the creation of Earthsea.
The Creation of Ea also gets mentioned at the end of the book. When Vetch considers what Ged just went through with his shadow, he thinks to himself the lines that are the epigraph (10.73). Thanks, Vetch! Your connection between Ged and the shadow and the epigraph totally helped us out. That is, the epigraph is kind of the key to the whole book, which is the idea that opposites are necessary to each other. As in, if you don't have darkness, you can't tell what's light. This issue comes up a lot in this book, in different ways; for instance, when Ged talks to Vetch's sister about how words need silence (9.80).
But here's a question: how does it change your experience of the book when you read this epigraph first? Do you know how the story is going to end? Or do you only realize the importance of this epigraph once you've finished the book? That's what happened to us, which seems like the same thing that happens to Ged: everyone tells him what he needs to know (keep the Balance), but he doesn't understand it until the very end.