In "To Kill a Mockingbird," Arthur "Boo" Radley stays inside because of the emotional and psychological damage that his father has wreaked up on him since childhood. Scout and Jem spend a lot of time imagining about his life because he is their neighbor but hardly ever appears outside.
At the end of the novel, when Scout is attacked on her way home from the food pageant, Boo leaves his house to save her life. This is one reason why Boo stands out as one of the "mockingbirds," or innocent forces for goodness in the book. Scout and Jem had reached out to Boo by leaving gifts for him in the knothole of a tree, and this comes back to help at a crucial point in the novel.
The other "mockingbird" in the book is Tom Robinson, a black man who was guilty of nothing more than kindness to a white girl. However, the girl's own lack of stability, and the cruelty of her father, leads her to falsely accuse Robinson of sexual assault, and because of the harsh realities of racism in that time period, the all-white jury found Robinson guilty, and he died in prison for "trying to escape."