Roll of Thunder, Hear My Cry takes place in rural Mississippi, where there are places with quaint names like "Strawberry." We never know the name of the town where Cassie and her family live. What we do know is that the predominant color of the land is red. There's red dust, red mud and red dirt, all of which liberally coat the Logan children each day on their way to school.
The story takes place in 1933, during the height of the Great Depression. This accounts, in part, for why so many of the characters are poor, and provides an account of how blacks fared during the Great Depression (since most of the more well-known novels set during this time feature white families: for example, The Grapes of Wrath).
But the characters in the novel aren't just poor because of the Depression. They're also poor because of racial inequalities in America, and particularly in the South. Keep in mind that we're way before the Civil Rights movement, and still firmly in the era of open racism and segregation.
A major problem is that sharecropping keeps the black characters down economically. These farmers had to pay rent for their land and also give up a percentage of the profits that they made on their crops. The system was rigged so that it was impossible for farmers like the Berrys and Turners to ever get ahead. Most had really no hope of ever escaping this level of poverty.
Remember: the Logans have it good by comparison. They at least own their own land and have some level of autonomy. But, because of their skin color, they're still at the mercy of unscrupulous people like Mr. Granger, who has Mama fired and who constantly schemes to take the Logans' land away from them.
Times are starting to change, though, in the 1930s. Black people are more upwardly mobile (think about Uncle Hammer and his good job in Chicago that allows him to afford a nice car), and educated (Mama and her teacher training), especially in the North. Great, right?
Not so much. The Southern whites reacted to this change by going after any African Americans that they saw as rising above their stations. Taylor uses the "night men" to help us feel the terror of these times—the world was changing, and not everyone was happy about it.