Tone is the emotion or attitude a poet conveys in a poem. In "Harlem," Hughes's tone if one of bitterness and despair. He attempts to communicate how the Black community in Harlem feels about their situation.
In 1951, when Hughes published the poem, Harlem was a depressed area of badly substandard housing into which Black people were crowded because of segregation. Once a vibrant center of the Black cultural life, the economic collapse of the Great Depression left the area decaying, without jobs or resources.
The context of the poem, written just as the Civil Rights movement was in an early state of rebirth but had not yet made gains, is that Black people have dreamed for a long time of having better lives, such as those white people were rapidly gaining in the post-war prosperity. But Black dreams, as the poem's speaker notes, are "deferred": Black people are endlessly told that their rewards will come later.
Using a series of unpleasant images, Hughes tries to convey what it feels like to constantly have one's dreams thwarted and delayed. He likens it to a raisin left so long in the sun that it dries out, to rotten meat, and to something crusted over and sickeningly sweet. He wonders in the end if such a state of despair just "sags like a heavy load" or if it will "explode" into rage and violence. The negative tone of the poem is meant as a wakeup call.