The key section is:
Oft of one wide expanse had I been told
That deep-brow’d Homer ruled as his demesne;
Yet did I never breathe its pure serene
Till I heard Chapman speak out loud and bold:
The sense of this quatrain is that Keats had read other translations of Homer before reading the one by Chapman. Given his period, the most popular translation would have been the one by Alexander Pope, one that transforms the Homeric poems into stately, measured, regular heroic couplets. The famous classicist Richard Bentley said on the publication of Pope’s Homer, “A very pretty poem, Mr. Pope, but you must not call it Homer.” Chapman’s rougher more irregular versification conveys a majesty and wildness that Keats, himself a Romantic, would have preferred to the Augustan rationality of Pope’s version.
Keats compares the experience of poetry to that of travel to new and unexplored lands in the following stanza.