The speaker opens this poem with an apostrophe to an alluring figure: “Golden-tongued Romance” herself, a personification of fantastical tales of chivalry and magic. A “Syren” (that is, a spirit of temptation) with a melodious voice, decked out in “fair plume[s],” Romance is both attractive and a little dangerous, threatening to lure the speaker onto the rocks of triviality. The speaker conjures her up only to banish her in favor of a darker, richer kind of poetry: the poetry of Shakespeare’s King Lear. His vivid description of her, however, suggests he knows her pretty well. Romance has been his longtime companion, but he’s ready to move on; even still, he sends her packing on a tide of praise for her beautiful voice.
He then makes a fresh apostrophe to the figures he hopes will guide him in his new works: Shakespeare himself (whom Keats saw as his “Presidor,” his literary hero and forebear) and the “Clouds of Albion,” the clouds that hang over Britain. These together, he says, are the “begetters of our deep eternal theme,” the sources of the ideas that will inform his work now: ideas about the nature of life, the shape of the human soul, and the meaning (or meaninglessness) of suffering.
This “eternal theme,” in other words, comes both from Shakespeare himself and from the English landscape Shakespeare and the speaker share across time. Praying to Shakespeare and the clouds at once, the speaker asks for support in becoming not just a great poet, but a great English poet, a master of his native language as well as the “deep eternal theme.”
By framing these two ideas as apostrophes, the speaker draws on a grand old poetic tradition of calling on muses, gods, or spirits to guide one's pen. (See, for instance, the first lines of Paradise Lost, another work Keats held in special regard.) A writer’s writer, this speaker can't think of a better muse than the immortal “Chief Poet” of them all.