What does the title mean literally and metaphorically in the book?

Atticus said to Jem one day, 'I'd rather you shoot at tin cans in the back yard, but I know you'll go after birds. Shoot all the blue jays you want, if you can hit 'em, but remember it's a sin to kill a mockingbird.'

That was the only time I ever heard Atticus say it was a sin to do something, and I asked Miss Maudie about it.

'Your father's right,' she said. '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.'" Page 90

In addition to bearing the title of the novel, this passage demonstrates yet again how similar Atticus and Mrs. Maudie are. Both agree quite strongly that it is a sin to kill a mockingbird, an animal symbolic (a metaphor) of Boo Radley and Tom Robinson, as neither has caused harm, and prove only to have pure hearts.