The poem is a variation on the Japanese form of the haiku, a very short poem divided into three sections with a certain number of syllables in each section. In English, haikus are often written as three-line poems. The first line has five syllables, the second seven, and the third five again. The haiku is really short, but it packs a big punch.
It’s hard to achieve the same effect when writing in a language other than Japanese, which is probably why Pound doesn’t follow strict rules in creating this short poem. For example, Pound’s poem has two long lines instead of three short ones. So, how can we tell that the poem is a haiku? For one thing, the Japanese version often features a contrast between two events or images; Pound’s poem clearly contrasts the two images of the faces in the crowd and the petals on the bough. The poem attempts to fuse these images into one. Second, the Japanese haiku usually has a word that lets the reader know the season of year. It’s not super-obvious, but Pound’s poem also has a word ("petals") to indicate the season – springtime – in which it takes place.