Harper Lee explores a number of themes in her novel To Kill a Mockingbird. One of the most obvious is that of prejudice. The racial prejudice that exists in Depression era Alabama comes through loud and clear in the story. Segregation is still in full swing. African-Americans live on one side of town, whites on another; blacks attend a separate church. Scout mentions no black children at her school and, indeed, there would be no such mixing in the 1930s. In the case of Tom Robinson, nearly the whole town assumes he is guilty of the rape charge despite the fact that his accusers, Bob and Mayella Ewell, come from the most disgraceful white family in Maycomb. The jury is likewise biased, and they disregard the evidence presented by Atticus that should clear Tom's name.
Boo Radley suffers from another form of prejudice. Though he is never seen by any of the townspeople (except for the doctor and members of his own family), Boo is blamed for most unexplainable crimes that occur in Maycomb because of his mental instability. He is thought to be a night prowler and animal mutilator, and few people seem to have any sympathy for the ill treatment his family has displayed toward him. Boo and Tom are in fact quite similar--both are kind and gentle and innocent of any of the accusations made against them--but only a very few people will ever realize this, and Maycomb's attitudes about them will not change significantly.
Check out the eNotes link below for other themes in TKAM.