The title of the novel "To Kill a Mockingbird" by Harper Lee, has very little actual relation to the plot of the story, however, it carries a significant amount of symbolic weight in the book. The plot tells of innocence being destroyed by evil, the "mockingbird" comes to represent the idea of innocence. Thus, "To Kill a Mockingbird" is to destroy innocence. The innocent are chiefly the victims of society in the novel. The symbol of a "Mockingbird" is used as a metaphor for the good and innocent, such as Boo Radley and Tom Robinson, who are as a result of the evil encountered in the prejudice people of Maycomb County, destroyed.
The novel suggests that "It is a sin to kill a mockingbird", sin being something that is morally wrong, meaning that although it isn't against the law or illegal to persecute an innocent, it is still immoral. Morality means essentially the goodness or badness of people based on their values and attitudes towards things. Your attitudes are what you feel about something based on your values; your opinion comes out of and is driven by the values you believe in. Being immoral is on the contrary to moral principles.
The main mockingbirds of the novel are Tom Robinson and Boo Radley (Jem being a more minor "mockingbird"), being that they were both innocent and got destroyed. However, Jem still fits under the definition as he is an innocent who was "damaged" due to the evil.
Tom Robinson is a Negro who lives just on the outskirts of town in a small Negro colony. He is respectful and righteous, with only a good heart with good intentions. Tom Robinson is a symbol of goodness, as is the entire Negro population in Maycomb County, who are portrayed as idealists without any bad in them. They are however, greatly looked down upon having no status and considered worse than the worse of white people, such as the Ewells. They are considered scum by white people just because they are black skinned.