You’re referring to Ezra Pound’s essay “A Retrospect,” containing ideas first expounded by F. S. Flint in his essay “Imagisme” from the March 1913 edition of Poetry alongside Pound’s “A Few Don’ts by an Imagiste.” These works discuss three formal guidelines for a new strand of modernist poetry called Imagism:
- Direct treatment of the “thing” whether subjective or objective.
- To use absolutely no word that does not contribute to the presentation.
- As regarding rhythm: to compose in the sequence of the musical phrase, not in sequence of a metronome.
Before literary modernism, English poetry tended to employ fixed meter, or set rhythmic patterns. Imagists were interested in shedding these poetic traditions, exchanging the vague optimism and verbal ornamentation characteristic of Romantic literature for historical accuracy and concise language. They often employed free verse, poetry without fixed rhythm or rhyming schemes, in an effort toward a new kind of poetry composed of clear and precise visual images.
Your question regards the third point, where Pound suggests “compos[ing] in the sequence of the musical phrase, not in sequence of a metronome.” In Pound's view, a musician wouldn’t set the tempo (speed) of a piece before writing the melody or alter the melody to adhere to a rhythmic tradition, so why should a poet choose to emphasize meter over content? He encourages the budding Imagist to imitate composers of Western Art Music, who tend to let the musical phrase (the main motif or melodic idea) guide the rhythm (meter) of a piece rather than fitting melodic ideas into preconceived rhythms.
Although Pound recommends emphasizing content over rhythm, he doesn’t suggest that the poet ignore the rhythmic content of a poem altogether. He believes “the rhythmic structure should not destroy the shape of your words, or their natural sound, or their meaning.” He encourages writers to abandon superfluous language in poetry and focus on constructing images with sharp, accurate words. Although Pound generalizes about the ways in which music is composed, his musical metaphors helped shape modern poetry.
Pound, Ezra. Pavannes and Divagations. New York: Knopf, 1918.