This is a simple poem that uses economy of words, so I will try to do the same.
One of the themes of this poem is that life is short, and we need to capture the moments. The poem is only fourteen words long. It captures the moment of the metro station with a beautiful simplicity. By choosing the bold imagist style (see the second link), Pound comments on the brevity of life.
Another theme that I take from this poem is that imagination is equal to reality. If you look at the semicolon between the two lines as making them equal halves of the same sentence, two independent clauses, then the literal “faces” become figurative “petals” and reality and imagination are equal.
Despite being a very short poem, Ezra Pound's "In a Station of the Metro" explores a number of major themes. First of all is the theme of the modern world. This is shown most clearly through the poem's setting: a metro (underground) station in Paris. Pound's speaker is struck by the number of people he sees in this busy station, prompting him to compare the faces to "apparitions." This not only creates a strong visual image but also makes an important point about the modernization and growth of big cities like Paris: people are disconnected; they are just blurred faces in a crowd.
Secondly, Pound also explores another theme, that of the natural world. To show this, Pound uses a metaphor in which he compares the people in the station to wet petals on a "bough" ("a branch of a tree"). That these petals are "black" suggests that modernization, seen here through the Paris metro, has weakened society's relationship with nature. Moreover, by describing the "bough" as wet, Pound argues that this disconnection from nature is a bad thing: it has, quite literally, dampened people's spirits.