In Harper Lee's "To Kill a Mockingbird," Boo Radley becomes for the children a prototype of the bizarre recluse who is to be feared. For the town of Maycomb, Alabama, he is simply the stereotypical odd recluse with which many Southern towns were acquainted since there were few mental institutions where such people could be "sent off."
The mockingbird mentioned in the title becomes symbolic of Tom Robinson and Boo Radley both since, as Miss Maudie instructs the children, mockingbirds never bother anyone;
...they just sing their hearts out for us. That's why it's a sin to kill a mockingbird.
As the mockingbird, Boo tries to bring enjoyment to the children while displaying his love for them by repairing Jem's pants when he rips them on the Bradley fence, and by leaving little gifts for Jem and Scout. Certainly, with his tenderness and concern for the children, he acts as a foil to Bob Ewell, whose vindictiveness against Atticus Finch is turned upon the innocent Scout and Jem as they return from the halloween pageant. Of course, the significance of the title becomes evident with Boo's heroic gesture. For, as the sheriff tells Atticus, it would be a shame/"sin" to incarcerate Boo/"a mockingbird" for having killed Ewell.
Boo Radley's presence in the novel acts as a catylst for the motifs of superstition and appearance, motifs which play into the maturation of the narrator, Scout. As she matures, Scout realizes that the "haints" and the bizarre Boo Radley are not what she has believed. Instead, Scout sees that Boo is a delicate person who must hide from the harsh world; yet, he is brave and protective:
Atticus was right. One time he said you never really know a man until you stand in his shoes and walk around in thhem. Just standing on the Radley porch was enough.
His courage in coming out to defend Scout and Jem exemplifies how misunderstood Boo has been. So often in a small society such as Maycomb do people prejudge a person such as Boo, believing him malevolent because they fail to understand them. In reality, Boo is a true hero.
The book "To Kill a Mockingbird" was written at a time when most poor families took care of their own children with disabilities. It was not that uncommon to keep the children or adults in the home setting limiting them from venturing about. Mr. Radley personifies this attitude when he tells the judge he can not send his son to prison even after he stabbed one of them in the leg. Boo is a stereotype of a person with a simple mind (some type of disability).
By keeping Boo shut away from the others, speculation and stories develop around the town about Boo. He is the unseen boogie man and the house is the typical haunted house. He represents fear of the unknown which later parallels with the townspeople's views of black men, the fear of what they do not know (Prejudice).
Boo is also an innocent person. He does not hurt anyone in his adulthood. He stays to himself, but he wants to establish some kind of touch with the outside world. He gives the children small treats in the tree hole until his brother cements it up. He even follows the children at night and ends up protecting them from Bob Eewell. He kills Mr. Ewell to save the children. He is the mockingbird that is saved.
I think that Boo is a catalyst for change. If he had gone to trial he probably would have been convicted just like Tom had been. People in the town already had a preconceived notion of who Boo was and what harm he could cause. Their prejudice would have most likely resulted in his conviction just like Tom' had. Heck Tate makes a decision to cover the incident over by saying Mr. Ewell fell on the knife. He has set change in motion.