The apparition of these faces in the crowd;
Petals on a wet, black bough.
Given the economy of words used in this poem, Pound creates a striking image of a place and moment: the Metro station. The use of the word "these" in talking about the faces in the crowd indicates specificity. The faces you imagine in the metro are not the faces Pound writes about. He saves a moment he experienced, or imagined, and kept it to himself. He likened them to "petals on a wet, black bough" giving us some idea, but not a full picture. And yet, the scene is complete, for we know that a fuller scene exists. We are simply not privy to it.
In discussing extremely short form poetry, there is little to say, at times. And I think that is part of the point. Pound presents a scene that we can picture, but never picture fully. The beauty of the words themselves is enough to let us know that these little moments of life are meaningful, and carry weight beyond their apparent brevity. So too does the poem. Short form poetry, at its best, presents poems that are in structure and form their function. There is no divide between the two, ideally, and that is presented perfectly here.