In chapter 23, Atticus explains to Scout what will most likely happen to Tom if they lose the trial. He'll probably be sent to the electric chair for a death sentence because rape is a capitol crime in Alabama.
Before Jefferson's death, the chair is paraded around town on a truck. White people mock the chair and mock Jefferson, while african americans look at the chair in fear or consideration. This represents the inequality between blacks and whites, and the effect the thought of the execution of an innocent black man has on each race.