There are two important factors at work here. The first is that Jem has just made a narrow escape from the Radley house after going back to retrieve his pants. We don't know until later in the book, but Jem's pants were sewed up - someone fixed them for him and laid them across the fence. Jem is quiet and moody at this point in the story because he is trying to reconcile the popular view of the Radley's (namely, they are weird, even monstrous) with his own experience (one of them, probably Boo, fixed his pants). But perhaps more importantly, Atticus is trying to teach Scout empathy. He wants her to understand what it is like to see the world from someone else's perspective. This will become central to the novel in several instances, and right now, Scout seems to begin to understand. If the quote continued, it would show Scout trying to live the experience of going back to the Radley house late at night, something that would terrify her. She not only sees the event through Jem's eyes, she gains a new respect for her older brother in the process. Atticus has taught her well.