A metaphor in a poem, the same as in other types of writing, is a form of figurative language in which something is described as if it is something else--not compared to something else using words such as "like" or "as," but compared through implication and imagery. For example, on a moonlit night, "he looked up at the pale silver disc in the sky." (This is one of my own, but you get the gist--the moon is not literally a silver disc, but appears to be one.)

You haven't specified a particular Langston Hughes poem, but Hughes certainly makes use of metaphor in his works, like most poets. In "Dreams," for example, he makes extensive use of metaphor throughout, describing life as "a broken-winged bird" in the absence of dreams to make it fly. Later, a life without dreams is imagined as a "barren field/frozen with snow."

A metaphor is a comparison between two things that focuses on their similarities. A metaphor does not use the words "like" or "as" in the way a simile, which also compares two things, does. A simple example of a metaphor is "You are my sunshine," which makes the comparison between someone you love and the way the sunshine makes our lives warmer and brighter.

Langston Hughes, a poet from the Harlem Renaissance and one of the most famous African-American writers, uses metaphor to draw connections between important cultural ideas. For example, in his poem "Mother to Son," the speaker, an African-American mother, compares her life to a staircase when she says "Life for me ain't been no crystal stair." This staircase, as she describes it, is full of splinters, tacks, and torn up boards. In this poem, Hughes is making the connection between the difficulties of her life and an old warped staircase. The mental image that we get of the staircase from the speaker's description helps us to understand how difficult her life has been.