In chapter 15, the neighbours gather in the Finches' front yard one Saturday evening to discuss what to do about Tom Robinson, whose trial will begin the following week. Tom is the black man falsely accused of raping the white girl, Mayella Ewell. Atticus is his defense counsel - a fact which arouses the hostility of many white people in Maycomb.
Scout remarks that this meeting is when the grim events surrounding the trial really begin for the family:
A nightmare was upon us.
When Scout first sees the men standing outside, she says that:
In Maycomb, grown men stood outside in the front yard for only two reasons: death and politics. I wondered who had died.
Scout realizes that the men are gathered there for some serious business, but she can't quite make out what, although she and especially Jem listen diligently to the conversation that follows.
The reason the men have come is because they're worried that others, particularly the Cunninghams from Old Sarum, might try to lynch Tom Robinson at Maycomb jail before the trial begins. Atticus dismisses the notion, but Tate points out that it's a real possibility: indeed, almost a certainty. He is proved quite right when, the next night, the lynching party does indeed turn up at the jail.
Although Atticus appeared rather disbelieving during the meeting in the yard, he has been sufficiently primed to go to the jail himself the next night, in a bid to protect Tom Robinson from what might happen. He is there defending Tom alone - until his children and Dill, anxious to see what he's up to, also turn up. Jem has more than an inkling that there might be trouble, which is why he follows Atticus to the jail, while the other two just tag along. However, Scout most unintentionally saves the day when, upon recognizing Walter Cunningham, the leader of the lynching party, she hails him cheerfully, not realizing his sinister purpose. Faced with her uncomprehending innocence, the men can only back off.
One encounters some irony when comparing these two incidents. Scout appears somewhat afraid of the men who come to the yard on Saturday evening, yet they turn out to be friends and neighbors: "people we saw every day," who have come to help Atticus by warning him about the lynching party. However, when there is real danger to Atticus outside of the jail, she doesn't realize it.