Much have I travell'd in the realms of gold (1)
The world of poetry is, for the speaker, like undiscovered lands rich in gold. More than that, because gold is found underground, Keats implies that most of what poetry has to offer is beneath the surface. You dig? (Sorry, we couldn't help ourselves.)
Oft of one wide expanse had I been told That deep-brow'd Homer ruled as his demense (6-7)
Calling Homer "deep-brow'd" doesn't mean he needs to pluck. Keats is trying to communicate the solemnity and power he sees in great poets by imagining Homer as a wise ruler.
Yet did I never breathe its pure sereneTill I heard Chapman speak out loud and bold (8)
To Keats, poets weren't sissies sitting on some hillside writing in notebooks. They were powerful bards (4) who spoke audibly and told grand epics (hence the "loud" and "bold"). Ironically, he shares this belief in a little poem he wrote in his notebook.