What is a Mockingbird known for?

Arthur "Boo" Radley

According to Atticus Finch, the mockingbird is an example of innocence and goodness, and so should never be killed. According to Scout Finch then, Arthur "Boo" Radley is also an example of someone, who, though at times seemed frightening to the children, was really as harmless as a mockingbird.

Scout says that to punish "Boo" for the way he saved her and her brother, would be like "killing a mockingbird."