Symbolically, Boo Radley represents the innocence of childhood. When the children are younger, before they learn how the world really works, Boo Radley is their resident monster. As they get older, they lose their innocence when they learn about poverty (though the Cunninghams and Ewells) and racism (through the trial of Tom Robinson and people’s reaction to it). Poverty and racism are the real monsters, and as they are revealed Boo Radley becomes less of a monster. He begins reaching out to the children by leaving them the presents in the tree. He protects Jem from punishment by sewing his pants, and he protects Scout from death when Bob Ewell attacks her.
Boo Radley becomes less and less mysterious throughout the book as the children age and mature. At the end, he comes out. This is the point at which they realize that there are good people and bad people in the world but they are not the ones they thought. Racists are the real monsters. Boo, one of the story’s Mockingbirds, is actually the innocent one. He has been victimized by society, and this is the result of severe abuse from his parents. He is socially stunted as a result.