“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.” - Miss Maudie

The mockingbird is used as a symbol of innocent benevolence throughout the novel.

When Scout asks Miss Maudie about mockingbirds, her response reveals on of the underlying themes of the novel. She explains carefully that Mockingbirds never do anything that causes any harm, so it is immoral, even a sin, to kill one. Scout understands the basic concept, but fails to make the connection between the lesson and the people around her.  

The wisdom behind this quote is best modeled by Arthur "Boo" Radley and his horrific situation. Throughout the entire novel, Boo does not do a single malicious, harmful, or remotely dangerous act. He does quite the opposite, in fact, helping Jem and Scout whenever he can; he took the time to sew Jem's pants together, to place a blanket around Scout's shoulders on a cold night, and save two children from being murdered by a drunken lunatic. Arthur Radley becomes a symbolic mockingbird through his helpful and benevolent acts. The lesson to never kill a mockingbird is again shown when Sheriff Heck Tate goes out of his way to ensure that Arthur Radley would not be pursued for his defensive killing, and is allowed to go on living in the shadows, helping people in whatever way he can.