While To Kill a Mockingbird by Harper Lee is not a particularly symbolic novel, the mockingbird is an inescapable symbol which both Miss Maudie and Atticus try to help Scout understand. After Atticus makes his statement about killing bluejays (or other birds) but never killing mockingbirds, Scout feels the need to question her father's edict, mostly because it is so unusual. She asks Miss Maudie about Atticus's perplexing statement, and she gets an answer she can begin to understand.
“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 . . . but sing their hearts out for us. That’s why it’s a sin to kill a mockingbird.”
So, a literal mockingbird is a creature who sings beautiful music for the benefit of others and never does harm to anyone. A figurative mockingbird is someone who is innocent, who does kind things for others without asking for anything in return. Those kinds of people, according to Atticus, deserve to be protected. Anyone who tries to hurt them is committing a sin, because these kind-hearted people have done nothing but make the world a better place for those around them.
In this novel, two characters could be considered mockingbirds, and Harper Lee makes sure we make the connection. The first is Tom Robinson, a man who did nothing to deserve trouble except try to help a young girl who seemed desperately lonely and seemed to need his help. In exchange for that innocent act of kindness, he is accused of rape and will eventually pay the highest price for that kind deed. This is why Atticus defends Tom. Mr. Underwood, who has no real love for the black people in town, describes Tom's death as being similar to “the senseless slaughter of songbirds by hunters and children.” This is a clear argument that he is an innocent who did not deserve to die.
Later in the novel, we know that Scout has understood this important concept about mockingbirds when she tells her father that doing something to Boo Radley would be“sort of like shootin’ a mockingbird.” Like Tom, Boo has never done anything but act with kindness to others, especially the Finch children, and she understands that she must do her part to help protect him.
The entire premise of the accusations against Tom and Atticus's decision to "really" defend him is based around this theme of killing mockingbirds. It is a lawyer's job to defend his client no matter what; it is even more important when that client is unequivocally innocent but being prosecuted simply because he is black. Such an injustice should not stand undefended because it is a sin to kill a mockingbird.