Hughes's narrator uses five similes to ponder what happens to a dream deferred (deferred means put off or delayed). (A simile is a comparison of two unalike things using the words like or as.) First, he asks if the deferred dream dries up "like a raisin in the sun," withering and shrinking up. Next, he asks if it "fester[s] like a sore" and then runs (with pus). Third, he wonders if it will begin to "stink like rotten meat" or, fourth, "crust and sugar over— / like a syrupy sweet." Fifth, he suggests that it might sag "like a heavy load," weighing the dreamer down, even depressing them.
In the final line of the poem, however, Hughes uses a metaphor, a comparison of two unalike things where we say that one thing IS another; metaphors are understood to be stronger than similes. Further, the metaphor is contained in a line that stands alone, like the initial question it answers. Finally, it is italicized. Therefore, it is emphasized in three different ways. In it, the narrator asks, "Or does it explode?," comparing the dream to something that can explode, like a bomb. A bomb does a great deal of damage, so this metaphor implies that the dream deferred can harm not only the dreamer but many others as well. Perhaps whoever is forcing the dreamer to wait on fulfilling this dream will be injured by its deferment too. We might begin to consider what this dream could be. The fact that the author is African American, the fact that he wrote the poem in 1951 (before the Civil Rights Era), and the title provide clues that the "dream" might be a dream of racial equality. If whites continue to put off or delay this dream, Hughes seems to suggest, it will not simply harm those African Americans who want to achieve the dream but also those whites who prevent them from doing so. Those who oppress will be as damaged by the dream's effects as those who are oppressed, because a bomb injures anyone around it (unlike a dried-up raisin, a sore, a bad smell, and so forth).