In Langston Hughes' poem, "Harlem" the speaker is not necessarily a specific person—it could be Hughes, but we can also assume the speaker is any dreamer: however, with the poem's title, and the mission present in Langston Hughes' poetry (of portraying the plight of blacks in America), the piece speaks specifically to a black dreamer who has not been allowed to dream—has not been allowed to believe he has the right to dream and see his dreams come true.
The poem begins asking about the "dream deferred," alluding that for an individual, the loss of a dream—or one not realized—leads to something that describes rot or decay...
...representing the dream (or the dreamer's) fate
Hughes presents several phrases of this kind with the following imagery:
…dry up / like a raisin in the sun?…fester like a sore— / And then run?…stink like rotten meat? Or crust and sugar over— / like a syrupy sweet?
The poet offers yet another image, where the dream becomes like a burden bearing one down, to be carried around like a stone:
Maybe it just sags / like a heavy load.
With an image of the slave's lot in life—burdened and beaten under the yolk of white oppression for so long—ironically the black man, even while "free," is still carrying a heavy burden. All of the imagery in the poem so far suggests what may happen when a dream is not fulfilled.
It suggests an effect on the individual in that the images are unpleasant and repulsive, but not dramatically life-altering. However, the last line expresses a sense of violence—perhaps we can acknowledge it is even prophetic, asking what if that "dream deferred" explodes? For with an explosion, the threat is much greater, and the explosion affects more than one person.
It is suggested that this explosion is what eventually occurs when someone says, "no." The world witnessed the organized Civil Rights Movement of the 1960s when the dream promised by Abraham Lincoln's Emancipation Proclamation had not been realized; the poem's image of a "dream" becomes prophetic of Martin Luther King, Jr.'s galvanizing "I Have a Dream" speech.
The poem introduces the idea of a person's dream unfulfilled. The poet ends the poem by wondering how long it can remain a quietly painful disappointment? And when will it become so heartbreaking that a person cannot hold the pain inside anymore? Ultimately, this is what happens with Rosa Parks, the Montgomery Bus Boycott, sit-in's, freedom rides, etc., and eventually the March on Washington.