Scout is alluding the the real start of the trial. Though the trial is still in the future, this really marks when Scout begins to lose some of her innocence as part of seeing Atticus wrapped up in the trial. Plus, she will see a more malevolent side of Maycomb than she has ever seen before.
The nightmare in Ch. 15, of course, is the Old Sarum bunch that come to lynch Tom Robinson, who is being held in the local jail. Of course, Atticus chooses to sit outside of the jail and make sure that Tom is safe.
The kids sneak out and go to visit Atticus just as the mob arrives.
Atticus attempts to reason with the mob, but, in the end, it is Scout that saves the day. This is important because she is able to make Mr. Cunningham stand in Atticus's shoes, so to speak, for a moment. He sees Atticus as a father and friend rather than just someone standing in their way. Their anger is defused and they leave.
Scout saves the day and shows the reader that she was embodied one of the novel's main themes - empathy.
However, this scene serves to foreshadow the rest of the nightmare that is to come - the trial, the injustice, the racism, and, ultimately, Bob Ewell's revenge.