At the beginning of "Harlem," Langston Hughes asks, "What happens to a dream deferred?" In the rest of the poem, he asks a series of rhetorical questions which provide possible answers for this initial question. He asks, for example, if the dream deferred will "dry up / like a raisin in the sun," or whether it might "fester like a sore." He also asks if the dream deferred might "stink like rotten meat." The implied answer to each of these rhetorical questions is "yes": a deferred dream will "dry up," and a deferred dream will "fester" and "stink" and "explode."
On the basis of this poem, Langston Hughes would likely say to someone who is thinking about putting off their dreams that they shouldn't. The poem suggests that dreams that are put off, or deferred, will never be realized, but will rather rot or be destroyed. Therefore, Hughes would probably advise such a person to embrace their dreams and act upon them as soon as they can, before they begin to rot and before they "explode."
Hughes also echoes this sentiment in another of his poems, entitled "Dreams." Hughes begins this poem with the line, "Hold fast to dreams." He also says that "if dreams die / life is a broken winged-bird." The implication here is that when a dream is allowed to die, then life becomes more ordinary, and more limited. Given that in "Harlem," Hughes implies that dreams deferred are likely to die, we can infer that Hughes would tell a person thinking of deferring their dreams that they shouldn't because if they do, their lives will become more ordinary and more limited.