What happens to a dream deferred?
Does it dry up
like a raisin in the sun?
Or fester like a sore—
Hughes is questioning what becomes of a dream that is delayed, tossed aside, or thwarted by external forces. 
 Though the poem is not explicitly political and deals with desires and aspirations in general, it ties in with Hughes' broader exploration of the African-American dream and the American dream as a whole. Blacks in America had few opportunities to pursue their dreams during the era when Hughes wrote this poem. 
 
 
 Today, the opening line of “Harlem” is so well-known that it has become cultural shorthand for African-American social mobility and the quest for opportunity. It may well have helped inspire the central metaphor of Martin Luther King, Jr.’s  “I Have a Dream”  speech. 
 Check out how Talib Kweli alludes to it in his song “Everything Man”: 
 
 Also check out Killer Mike’s allusion in “Anywhere But Here” : 

Harlem What happens to a dream deferred?