Metonymy is a figure of speech that uses a term to replace the name of something else that closely embodies the same qualities. Writers use this kind of figure of speech to help readers grasp meaning.
In To Kill a Mockingbird, one of Harper Lee’s primary themes is the prejudice that people engage in, especially with certain hot-button issues, like race. The reader sees this in the reactions of the people of Maycomb toward Atticus Finch as he defends the black Tom Robinson who is accused of raping a white woman. This was (and sometimes still is) a deadly problem in the South.
At one point in the story, a gang of white men from Maycomb (led by Mr. Cunningham) attempt to storm the Maycomb County jail, presumably to lynch Robinsion. Atticus, who is guarding the jail (unarmed), has a hard time holding off the men until his kids show up and Scout, in her innocence and persistence, shames the men into leaving. The next day, as Atticus discusses the event with Scout, he says of Cunningham:
Mr. Cunningham’s basically a good man . . . he just has his blind spots along with the rest of us.
Here the term “blind spot” is an instance of metonymy. Atticus does not really mean that Cunningham is physically blind. He means he is unable to comprehend the inappropriateness of his actions—he just cannot understand how wrong he is until Scout starts talking, which makes him realize what a good man Atticus is. Blindness is often associated with prejudice and hatred--the inability to mentally see the value of others who may be different from us.