The speaker does not refer to a specific dream. Rather, he (or she) suggests that African Americans cannot dream or aspire to great things because of the environment of oppression that surrounds them. Even if they do dare to dream - their grand plans will fester for so long that they end up rotting or even exploding. As critic Arthur P. Davis writes, "When [Hughes] depicts the hopes, the aspirations, the frustrations, and the deep-seated discontent of the New York ghetto, he is expressing the feelings of Negroes in black ghettos throughout America."
For an excellent source on English composition, check out this classic book by William Strunk, Jr. on the Elements of Style. Contents include: Elementary Rules of Usage, Elementary Principles of Composition, Words & Expressions Commonly Misused, An Approach to Style with a List of Reminders: Place yourself in the background, Revise and rewrite, Avoid fancy words, Be clear, Do not inject opinion, Do not take shortcuts at the cost of clarity, … and much more. Details of The Elements of Style by William Strunk, Jr. partially available online at . Note: William Strunk, Jr. (1869–1946). The Elements of Style was first published in 1918.