The night riders were white members of the community who went out at night terrorizing African-Americans who also lived in the community. In the story, the night riders were akin to members of the Ku Klux Klan (white supremacists) who wore white hoods and terrorized African-Americans. This group gained power in the southern states of the U.S. after the American Civil War.
The Logan children (in Roll of Thunder, Hear My Cry), in one particular scene, feared that the night riders were "coming to get them" to exact revenge for the "bus incident." In actuality, the children were not the specific aim of the night riders, but the effect of their efforts still had a fearful impact on the Logan children.
The reference to “night riders” in Mildred Taylor’s Roll of Thunder Hear My Cry is intended to evoke the desperate fears of liberated slaves and, later, of African Americans in general, of white terrorists, especially those associated with the white supremacist group Ku Klux Klan, or KKK for short. The KKK carried out many of its terrorist acts, like lynchings and the fire-bombings of homes and churches occupied by blacks at night. The darkness of the night is almost universally associated in the minds of many, especially the less-educated, which most slaves and former slaves obviously were, with terror, and so it was during the night that groups like the KKK sought to maximize the fear among their targeted audience: African Americans. This is why, in her novella about African American life in the Deep South, specifically, Mississippi, during the 1930s, Taylor’s young narrator, Cassie, refers to “visions of night riders and fire mixed in a caldron of fear” awakening her from sleep. Nightmares with their genesis in the real-life terrors with which blacks lived across the American South frequently included visions such as those described by Cassie. It is also why, when the white store owner Kaleb Wallace becomes angered with Cassie and her brothers, Little Man and Christopher-John, he immediately evokes images of the night riders who terrorized blacks in the darkness, usually while adorned in the KKK’s trademark white robes and hoods: “One of these nights, you watch, nigger ! I'm gonna come get you for what you done ! You just watch ! One night real soon. . .”
The phrase “night riders,” as pointed out, refers to members of the Ku Klux Klan and other white supremacist groups who decried the outcome of the Civil War and who violently rebelled against Reconstruction and any notion of desegregations. It also evokes, however, the use of horses in their night-time raids, as automobiles had not yet been invented when the “night riders” first appeared.