The answer to this question can be found in Chapter 18, which is when Atticus cross examines Mayella Ewell. Part of what he does is to prove that Tom Robinson could not have hit her on the right side of her face, as his left hand had been mangled beyond repair by a cotton gin when he was a boy. This piece of evidence is crucial, as it supports earlier comments that Mayella made that her father had beaten and assaulted her. Of course, when confronted with the truth, Mayella does everything she can to deny it and resist it, which loses her sympathy with the reader. Although she undoubtedly enjoys a pitiful existence, she seizes the opportunity to victimise somebody else just as she is victimised, and causes far more harm to others than she herself receives. However, note the following quote that comes from the final chapter of this book, which could be taken as one quote that presents the general theme of the text:
"When they finally saw him, why he hadn’t done any of those things... Atticus, he was real nice..." His hands were under my chin, pulling up the cover, tucking it around me. "Most people are, Scout, when you finally see them." He turned out the light and went into Jem’s room. He would be there all night, and he would be there when Jem waked up in the morning.
Even though they are talking about Boo Radley, the novel as a whole argues that humans need to "see" each other truly, beyond superficial prejudices and stereotypes, to understand that they can be "real nice." What Atticus does in the court room is to force the jury to "see" Tom Robinson as he really is, and as a man who is innocent. He does this by proving he could not have assaulted Mayella.