Much of the storyline in To Kill a Mockingbird centers around judgments, good or bad, and as the novel progresses, Scout learns that a person cannot judge by appearances or prejudices, but rather must strive to know the truth of the situation. Let's look at some examples.
Tom Robinson is a Black man accused of raping Mayella Ewell, a white woman. There is no real evidence to prove that Tom committed the crime. He swears he is innocent. Atticus completely demolishes the case against Tom and shows that Mayella and her father are lying, but the jury cannot see beyond Tom's race, and they convict him. They judge by appearance and prejudice, not by truth. Tom is devastated by the verdict, and even though Atticus vows to appeal, Tom tries to escape from prison and is shot and killed. A faulty judgment has essentially claimed a man's life.
Scout and Jem should be learning from this example, yet they, too, engage in judgment based on appearance and prejudice. The target of their judgment is Boo Radley. Boo has long been a recluse, and stories about him abound in Maycomb, each stranger than the last. The children are scared of Boo and are terrified that he will do something horrible to them if he catches them. Their fear doesn't even let up much when they start finding trinkets in the hollow tree, but it is joined by confusion. As the children grow a bit older, they are not quite so worried about Boo, but they still avoid the Radley place mostly out of habit but still somewhat out of that old prejudice.
Scout and Jem do not discover the truth about Boo Radley until the night he saves them from Bob Ewell's vicious attack. Boo has cared for—and perhaps even loved—the children in his own way for a long time. He never, ever intended to hurt them. In fact, when he sees them in danger, he runs to their rescue, actually killing their attacker rather than allowing him to hurt the children. Scout learns then that she has long misjudged Boo Radley, and she becomes (for one evening) his protector and guide as he shyly navigates the world outside his home.