Chapter 17 takes place right in the midst of the trial of Tom Robinson, which is one of the key episodes of To Kill A Mockingbird. Atticus is defending Tom Robinson, a black man who has been accused of raping Mayella Ewell, a poor white girl. The three children, Jem, Dill and Scout, have come to the courthouse to watch the trial, which they consider one of the most exciting events to happen in Maycomb in a long time. Because the courtroom is overly crowded, they find a place to stand in the balcony, which is generally reserved for black viewers. They wind up standing next to Rev. Sykes, a black minister. This placement of the children also shows their social innocence, that they don't realize the social meaning of white people standing in the black gallery. Older, more experienced white people would not go there, but the children just see Rev. Sykes as a nice man and do not worry about race.
As the trial progresses, the victim's father Bob Ewell is called as a witness. He is portrayed as poor, dirty, ignorant -- someone who could be described as "white trash." On the witness stand, to accuse Tom Robinson, he testifies, "I seen that black nigger yonder ruttin' on my Mayella." The ignorant way that he accuses Tom causes shock and excitement in the courtroom, so that the judge takes several minutes to regain composure.
At this point, Rev. Sykes recommends that Jem take Scout home, presumably because she is too young to hear language like that, and for the "mature" theme of the trial. There is a brief discussion, and the children end up staying for the trial. This becomes a major point in their development.
This conversation also presents a nice contrast between "black" and "white" behaviors, which is a key theme of the novel. To Kill A Mockingbird was written at a time when racial tensions were high in the U.S., and it is interesting to note at this point in the book that the black character, Rev. Sykes, is the caring and mature one, while the white character of Tom Ewell is crude, vulgar and ignorant.