In To Kill a Mockingbird, Boo Radley is symbolic of a mockingbird. The mockingbird, as Miss Maudie and Atticus tell the children, is a creature that does no one harm, but simply sings to bring pleasure to others:
...they don't do one thing but sing their hearts out for us.
[This does not guarantee that others appreciate the value of the mockingbird: people like Bob Ewell who is so full of hate that he cannot see any beauty in the world. Tom Robinson is also symbolic of a mockingbird, and Ewell does all he can to see Tom destroyed.]
For Scout, Jem and Dill, Boo becomes a summer obession: they want to contact him, have him come out and meet them for ice cream. They role-play what his life must have been like until Atticus forbids them to continue. Boo is a source of mystery and even rumors from others in the community that have no compassion for the unfortunate Arthur "Boo" Radley.
However, regardless of how the children and the adults see Boo, Harper Lee allows Boo to not only be a source of curiosity and mystery, but she gives this ghost of a man the ability to have power over evil. It is, in fact, Boo who saves the lives of Jem and Scout when Bob Ewell tries to murder them. In this way, Boo is an unexpected hero. Almost like a mythological hero that shows up once every hundred years to accomplish a great feat, Boo comes out of his house to save the children he loves—for he has watched them as they have watched him. For the children, he will risk his life in a world that holds no place for him. And that quickly, he returns to that place again.
In this story, Boo Radley plays the role of a local hermit or recluse. He keeps to himself, never to be seen by society, throughout the entire novel until the last three chapters. The only evidence of his existence occurs as he relates to the children by wrapping Scout with a blanket in chapter 8, and by giving the children gifts through the knothole in the tree in his yard. At this, we cannot even confirm his existence for sure.
There are many local legends about this man and what caused him to keep to himself in the first place. It is rumored that he got into much trouble as a teen and to keep him from going to prison his father agreed to keep him confined to the house. Now, neighborhood legend has it that Boo eats the heads off of chickens and is a peeping tom who wanders the streets at night. None of these ever surface as true throughout the course of the novel.
These ideas about him serve the purpose of giving readers the understanding that when we judge others, what we think about them may not be true. Therefore we should be careful and respect people for who they are. We should not rely on appearances.
Boo is completely innocent of the charges of society. To the adults of society he represents the absurd or the strange or that which is different. To the children he represents the unknown, something they long to identify. To readers, he comes to represent truth, innocence, and the moral good.