phrase, not in sequence of a metronome.

A metronome is a device that ticks in a regular pattern. Pound explains that the "musical phrase" is a better way to think about grouping and arranging words in a poem, rather than abiding by expected and regular patterns of sound. He discusses the musicality of words and poetry later in the essay, reinforcing the notion that the musical rolls and waves of words in a poem are much preferred to the repetitive rhythm of words that imitate the sequence of a metronome.

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:

1. Direct treatment of the “thing” whether subjective or objective.
2. To use absolutely no word that does not contribute to the presentation.
3. 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.

One of Ezra Pound's three principles of Imagism was

As regarding rhythm: to compose in the sequence of the musical phrase, not in sequence of a metronome.

This principle was concerned with prosody. Pound was strongly interested in the different forms of possible rhythmic arrangements for verse, especially those of the troubadours and the early Anglo-Saxon and Italian poets. He saw the fluidity and musicality of their rhythms as more interesting than what he considered the metronomic regularity of much of Victorian verse. In many ways, he was part of the free verse movement that began in France in the fin de siecle, although his actual practice often was closer to extreme variations on a theme than what late–twentieth-century poets would term pure free verse.