The motif of the mockingbird is introduced in chapter 10 when Atticus tells Jem that "it's a sin to kill a mockingbird." Scout asks Miss Maudie what he means, and Miss Maudie explains that mockingbirds are inoffensive; they don't damage anything important to human beings, and they provide beautiful songs.
In chapter 14, Atticus defends Calpurnia when his visiting sister tries to convince him that it is time to dismiss her from his home. Atticus takes a stand and will not hear of it. He tells Alexandra, "she's a faithful member of this family and you'll simply have to accepts things the way they are." Calpurnia has done nothing out of turn, and in the mind of Atticus and his children, she is blameless. He will not have her turned out simply because his sister does not approve of her influence.
In chapter 16, Atticus again speaks up in defense of a blameless African American; again, it is to his sister, Alexandra. This time, it is Tom Robinson, his client. He tells his sister in no uncertain terms that he is not in favor of "preserving polite fiction at the expense of human life." Atticus has no intention of bowing to public opinion or disapproval about his role in defending Tom Robinson.
A third quotation that relates to the meaning Atticus intends to impart comes from Dolphus Raymond in chapter 20. He explains to Dill and Scout that one day, they will
cry about the simple hell people give to other people—without even thinking. Cry about the hell white people give colored folks, without even stopping to think that they're people, too.
Raymond's point aligns with what Atticus is teaching his children: to condemn, mistreat, and malign people simply for being who they are is immoral.