Ezra Pound's poem 'In a Station of the Metro' is a masterclass in the use of simile; it compares an urban image to one from Nature. The effect of this is to remind us that, although Nature is the poet's usual milieu, poetry may be found anywhere. Also, it makes us look afresh at something we have all seen: faces in a crowd. How do they look like 'petals on a wet, black bough? Colourful petals stand out on such a bough in the same way as faces do in the crowded metro.
The syllable count is also important: the 'petals' line is 7 syllables so overall the poem has an 8 - 12 - 7 syllable pattern recalling the haiku, which also has a longer middle line and only 3 lines. The haiku traditionally provides a snapshot, suggesting meaning rather than spelling it out. I think the poem is a meditation on individuality: each of these faces, even in this setting, stands out, each one is different.
Finally we should not forget that Pound uses the word 'apparition' to describe both these images. I suggest that whatPenguin Book of American Verse he suddenly sees is their beauty.