Harper Lee's use of symbolism in To Kill a Mockingbird is one of its defining characteristics. Even the title of the book is an example of symbolism; the mockingbird represents innocence. In the novel, Miss Maudie Atkinson 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.
When the people of Maycomb convict Tom Robinson of the rape of Mayella Ewell, they are "killing a mockingbird." In addition, Tom is killed attempting to escape prison and return to his family, which is clearly further use of the same example of symbolism. Tom was a kind, gentle person and did nothing to try to harm Mayella (or anyone else). He was a "mockingbird."
Arthur "Boo" Radley's persecution by those around him is another example of "killing a mockingbird." In actuality, Boo is a more scared of people than they are of him; he watches Scout and Jem with love and even saves their lives. He is innocent of the rumors spread about him and is unable to defend himself, but that does not stop people from talking about him as if he was a monster.