This poem narrates a memory of the speaker when he first read a translation of The Illiad and the effect that this text had upon him. He perceived the poetry so profoundly that this moment is compared to the excitement that explorers feel when they look upon a tract of land that they have never seen before. Note how Keats presents this simile:
Then felt I like some watcher of the skies
When a new planet swims into his ken;
Or like stout Cortez when with eagle eyes
He stared at the Pacific...
Clearly then the theme of this poem is the way that reading and literature allows us to travel in "the realms of gold." For Romantics, poetry was the true adventure, and the best poetry allowed you to enter new worlds of thrilling voyages and explorations. Thus the theme of this excellent poem is that of adventure through imagination and the way that literature is the key to enable us to engage in such flights of fancy and adventures, just as if we were actual adventurers ourselves.