Answer:
The mockingbird represents the innocent, harmless characters such as Tom Robinson, Boo Radley, and the children (especially Scout).
The title is taken from a quote in the book, where Atticus says "Shoot all the bluejays you want, if you can hit 'em, but remember it's a sin to kill a mockingbird." Later, Miss Maudie Atkinson explains that "Mockingbirds don't do one thing but make music for us to enjoy." This shows that they are innocent, and that is why Atticus forbids his children to kill them.