In Chapter 10 of To Kill a Mockingbird, Atticus tells Scout and Jem, “Remember it’s a sin to kill a mockingbird.” Scout is confused by her father's reference to sin, a word he rarely uses, and she asks Miss Maudie to clarify what Atticus means. Miss Maudie tells her that since mockingbirds do nothing other than sing, it's a crime to hurt them. In other words, mockingbirds are pure, innocent, defenseless creatures, so it is a sin to hurt them.
The mockingbirds in the novel are the characters who are in a weak position so that harming them is indefensible. Boo Radley is one of these characters. Although people are afraid of him, he is in reality a gentle man who is probably a bit mentally disabled. In the end, he proves that he is really stronger and more principled than people think when he saves Jem's and Scout's life.
In addition, Tom Robinson, the African-American man who Atticus defends, can be considered a mockingbird, as he is in a weak position in society and in the trial because of racial prejudice in Maycomb. It is a sin to accuse him of a crime he did not commit because he is in a weak (and innocent) state. Finally, Jem and Scout can be considered mockingbirds because they are children and innocent, so it is indefensible for Bob Ewell to attack them at the end of the novel.