Chapters twenty-nine to thirty-one are the final chapters in Harper Lee's novel To Kill a Mockingbird, and they are quite significant for many reasons.
First of all, we learn what happened to Jem's elbow, something she mentions in the first chapter of the novel but is only explained here. Obviously he injured his arm when he was fighting first to save Scout and then himself from Bob Ewell's knife attack.
Second, we learn that Atticus lives what he preaches and is prepared for his son to go to trial for killing Ewell, though we learn later that it was not Jem who killed Ewell. Despite what happened to Tom Robinson, he still has faith in the criminal justice system.
The third thing is a big one, something we have been hoping for since almost the beginning of the novel: we finally meet Boo Radley in person. We have heard about him, we have seen his handiwork, and we have heard all manner of stories about him, some true but most untrue. When Atticus draws attention to the man hiding in the corner, Scout gradually realizes who it is. Her first words to him are a memorable understatement: "Hey, Boo."
Finally, we experience kind of a grand reversal, when Heck Tate insists that he and the Finches lie about Boo Radley's role in Bob Ewell's death. Though Boo would certainly be found innocent in a court of law, the people of Maycomb would all want to lavish Boo with praise and gifts for getting rid of a nuisance and saving Jem and Scout. This, according to the sheriff, would be a punishment for the heroic Boo, and he thinks they should spare him that by lying and saying that Ewell fell on his own knife.
Atticus sat looking at the floor for a long time. Finally he raised his head. “Scout,” he said, “Mr. Ewell fell on his knife. Can you possibly understand?”
Atticus looked like he needed cheering up. I ran to him and hugged him and kissed him with all my might. “Yes sir, I understand,” I reassured him. “Mr. Tate was right.”
Atticus disengaged himself and looked at me. “What do you mean?”
“Well, it’d be sort of like shootin‘ a mockingbird, wouldn’t it?”
This is a striking couple of lines for two reasons. First, Scout does seem to understand, in a way that seems quite beyond her years, what her father and Miss Maudie meant when they said it was a sin to kill a mockingbird. Second, Atticus is doing something almost completely out of character for him--he is agreeing to participate in a lie. This goes against all of his personal principles as well as his unwavering commitment to the criminal justice and legal system; however, it also supports his belief about "mockingbirds" like Boo Radley and Tom Robinson. He believes they need to be protected, and this is the only way he sees to do that. He thinks about it for a time, and surely he is remembering what happened to Tom as he does so; he finally agrees to participate in this lie. It is a striking moment in the novel.