It's probably best to start with two of Harper Lee's most enduring statements, both coming as advice given by Atticus Finch to his children. One is that "it's a sin to kill a mockingbird." Atticus gives this advice after Jem and Scout receive air rifles for Christmas, and Miss Maudie explains that the mockingbird is an innocent creature meant only to "make music for us to enjoy." The quote is symbolic for several of the human characters who are punished unjustly during the novel, most notably Tom Robinson and Boo Radley. The innocent mockingbird is evoked throughout the novel, especially when death appears imminent.
Another of Atticus' famous quotes comes when he explains to Scout that
"You never really understand a person until you consider things from his point of view--until you climb into his skin and walk around in it."
It stresses the importance of tolerance toward others, the evils of prejudice, and how things don't always appear to be as they seem--three of the most important themes in To Kill a Mockingbird.
Other lessons to be learned from the novel include the different types of courage--both physical and mental--that people can display. Loss of innocence and how childhood events affect one's later years is shown by Scout in her adult reminiscences of the important events during the novel's two-plus years.