How does Lee introduce the significane of the book's title? What does it symbolize?

Lee introduces the book's title in Chapter 10 when the children receive air-rifles from their Uncle Jack. Atticus tells them they can shoot at the Blue-Jays or tin cans, but they must remember "it's a sin to kill a mockingbird."

Scout was confused about what he meant, but Miss Maudie went on to explain;

"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."

The mockingbird symbolizes innocence.