One of the main themes in the novel is it's a shame to kill a mockingbird. Mockingbirds are harmless and only contribute something beautiful. Throughout the novel Atticus tries to instill in Scout and Jem the importance of leaving people to their own lives. Jem and Scout are obsessed with making Boo Radley come out; however, Atticus again and again tells Jem and Scout to leave Boo alone. He is harmless. It's a shame to try and disrupt his life.
Tom Robinson is the other mockingbird in the novel. Bob Ewell Knows he can accuse a black man of a crime, even though he knows Tom is innocent. Bob uses Tom to cover up the mistreatment of Mayella, his daughter. Tom Robinson lives his life without disturbing anyone; he is harmless. The outcome, the guilty verdict, is a sin. It's a sin to kill a mockingbird.
When Boo Radley saves Scout and Jem from Bob Ewell Atticus wants to do the right thing in the eyes of the law and bring Jem to trial for the murder of Bob Ewell. However, Jem is not responsible for Bob's death, Boo is. Heck Tate said it would be a sin to bring Boo Radley "out". Leave Boo Radley alone, "Bob Ewell fell on his knife."
Atticus tells Scout that "it is a sin to kill a mockingbird," and Miss Maudie further explains that the reason for this is mockingbirds do no harm. They only "sing their hearts out" and make music for people to enjoy. Therefore, it is a sin to kill one.
Many characters in the novel represent "mockingbirds." These can be any people who go about their business hurting no one, but somehow end up being hurt by others for no reason. Tom Robinson is an example of this.
Possibly the best example for a mockingbird, and perhaps the most relevant reason for the title, is Boo Radley. This is a character who does not emerge until the end of the book for good reason. Boo does not want to be in the spotlight in any way. The children make up stories about him because they know so little of him. Therefore, when he comes out of his house to save Jem and Scout from Bob Ewell, he risks being exposed to people more than he has ever been, possibly in his entire life. If thy adults tell the truth about what happened, Boo will become the town hero. It would likely change his life forever. They would be, in essence, killing a "mockingbird." Boo was only doing what was right, and ultimately the adults decide that it is not fair to out him to everyone because to do so would be a sin.