Three main themes in To Kill A Mockingbird are prejudice, maturation, and courage.
Prejudice, or prejudging a person, is the main theme of the parallel storylines of the novel. In the first, the children prejudge Boo Radley without knowing him, deciding that he is a frightening bogeyman. Because of their preconceived notions, they find it hard to accept the ways he is reaching out to them in kindness, such as putting a blanket over Scout's shoulders on the cold night that Miss Maudie's house burns or mending Jem's torn pants. Likewise, the white community in Maycomb prejudges Tom Robinson, deciding he must be guilty of rape because he is a black man. Even though Atticus offers a fair defense of him that shows that Robinson could not have raped Mayella, racial prejudice means he is found guilty.
Unlike the racist whites of Maycomb, Scout and Jem mature over the course of the novel, coming to understand, after he saves their lives, that Boo is not a monster but a compassionate human being. Throughout the novel, as well, they are offered lessons that help them develop a more nuanced understanding of the world, such as when Calpurnia takes Scout and Jem to her black church and they witness the humanity and solidarity of the black community.
The children also receive repeated lessons in courage. They learn that Mrs. Dubose is a woman of great courage as she fights her morphine addiction before she dies. Scout also comes to understand Atticus's courage, both when he kills the rabid dog and, more importantly, when he stands up to his local white community by mounting a fair defense of Tom Robinson.