As the title of the novel implies, the mockingbird serves as an important symbol throughout the narrative.
In the novel, a number of episodes feature harmless and innocent people being attacked. Mockingbirds are identified by both Atticus Finch and Miss Maudie as examples of creatures that do no harm and are deserving of protection.
As Miss Maudie Atkinson explains, it would be thoughtlessly cruel to kill innocent creatures that "don't do one thing but make music for us to enjoy."
Part of the moral of the novel concerns the notion that the innocent should be protected by those who have the privilege, the power, or the will to protect them. We see this in Jem's protection of Scout, in Atticus defense of Tom Robinson, and in Miss Maudie's defense of Atticus. Scout also comes to the defense of the defenseless, especially at the novel's end as she walks Boo Radley home in the dark.
Regarding mockingbirds specifically, Jem and Scout are told never to shoot a mockingbird with their air rifles. More metaphorically, they are instructed about the moral imperative to never take advantage of people who are powerless and to refrain from abusing the rights of others.
Through these episodes, the mockingbird becomes a symbol for harmlessness and for justice.
It has also been suggested that Mayella Ewell is a Mockingbird in this novel as she was victimized by her father and she did not want to hurt anyone.
We know she was a nice person because she worked so hard to earn up all those 7 nickels to give to her syblings so that they could go into town and get ice cream.
1) The mockingbird symbolizes something which is harmless, and which brings pleasure and happiness.
2) The mockingbird is used as a symbol because it is something which should not be hurt, because it has done nothing to deserve being hurt, attacked, injured, or tormented in any manner.
3) To hurt a mockingbird is a terrible thing, precisely because it is defenseless and because it exists to bring joy to the world through its prettiness and its sweet song.
Boo Radley is an example of a mockingbird, because he does nice things for Scout and Jem. For example, he leaves chewing gum and interesting little toys for the children to find. To add to the children's delight, he hides the goodies in a tree, knowing that children love to explore little holes and niches in trees and things.
Atticus is another mockingbird. Except for the fact that he is deeply involved at the center of an extremely controversial and emotional court case, he pretty much minds his own business. He keeps to himself, is polite to the neighbors, reads the newspaper, stays out of trouble, and is generally reserved but cordial. He harms nobody. He brings only good to his community. He has only the best intentions.