The mocking bird represents the innocent. Two who are like the mockingbird are Tom Robinson and Boo Radley. Both have not set out to harm anyone. Boo tries to please the children by leaving small gifts, while Tom attempts to help Mayella and ends up in court for his efforts.
The theme is especially strong in both chapters ten and thirty. In chapter ten, after the children are presented with air rifles, the are cautioned against shooting mocking birds. As Miss Maudie explains, the "don't do one thing but make music for us to enjoy". Atticus further states they are not harmful to anyone else. They don't tear up people's property or make nuisances of themselves. The mockingbirds are silent in the showdown with the rabid dog, as Atticus shoots it.
In chapter thirty, Scout reasons with her father not top reveal Boo Radley's part in the Ewell killing, because it would be "like killin' a mockingbird". Scout knows that the town would refuel their scrutiny and humiliation of Boo, who was out only to protect the children. Nothing good would come of revealing his assistance.
When Atticus gives Jem and Scout air-rifles, he tells them they can "shoot all the bluejays [they] want" but that "it's a sin to kill a mockingbird."
Miss Maudie explains Atticus's statement to Scout by saying, "Mockingbirds don't do one thing but make music for us to enjoy . . . they don't do one thing but sing their hearts out for us. That's why it's a sin to kill a mockingbird."
The mockingbird most obviously represents the innocent who is not protected by society. Just as a careless shooter might hurt an innocent mockingbird, so are innocent people hurt by those who are careless or who have evil intentions. Two of the major "mockingbirds" in the novel are Boo Radley and Tom Robinson.