In Chapter 10 of To Kill A Mockingbird, Harper Lee first introduces the quote that she chose for her novel's title. Here, Scout recalls Atticus's advice to Jem, after Uncle Jack gave the children air-rifles, that he could shoot at tin cans and bluejays in the backyard, but that Jem should remember "it's a sin to kill a mockingbird."
Puzzled by this, Scout asks Miss Maudie for her explanation of Atticus's advice. She tells Scout that:
Mockingbirds don't do one thing but make music for us to enjoy. They don't eat up people's gardens, don't nest in corncribs, they don't do one thing but sing their hearts out for us. That's why it's a sin to kill a mockingbird.
Upon reading the rest of the novel, readers come to realize that a mockingbird is a symbol for a person who is innocent but unjustly persecuted.
One example of such a character is Tom Robinson; he is a good, caring, innocent family man who is wrongfully convicted of rape. Further, Boo Radley, whom the children unfairly labeled a "malevolent phantom" (and whom the citizens of Maycomb unfairly persecuted because of his failure to participate in their society), is another example of a character who can be compared to a mockingbird. In fact, at the end of the novel, Atticus tries to explain why he and Mr. Tate come to the agreement to lie and say Bob Ewell fell on his own knife. (In doing so, they are protecting Boo Radley from unwanted attention in Maycomb.) When Atticus asks Scout if she understands, she says, "yes sir, I understand. Mr. Tate was right." When Atticus questions her response, she says
Well, it'd be sort of like shootin' a mockingbird, wouldn't it?"
Obviously, Scout has learned that it is neither fair nor right to bother people who do no harm to others. Tom Robinson and Boo Radley are the clearest and most obvious characters who embody this idea and can be labeled as figurative mockingbirds.