Epigraphs are like little appetizers to the great entrée of a story. They illuminate important aspects of the story, and they get us headed in the right direction.
Have you ever heard the term "negro spiritual?" Well, these "spirituals," or religious-based songs, were originally sung by slaves in the fields, and then handed down through African-American churches. We couldn't find "Roll of Thunder, but here's one you might be familiar with.
Many of these spirituals brought comfort to slaves during their captivity, but some were also used to communicate defiance. The song used in the book is more rebellious, and points to the blacks' refusal to put up with white domination anymore. Even though they might be beaten down (literally), they refuse to "turn 'round" (or go back to being subservient).