In chapter 10, Atticus tells his children that it is considered a sin to kill a mockingbird. Miss Maudie elaborates on Atticus's statement and explains his reasoning by mentioning that mockingbirds do nothing to harm or annoy anyone, and simply make beautiful music for people to enjoy. Mockingbirds are also vulnerable, defenseless beings, which is why people should not shoot them. Throughout the novel, mockingbirds symbolize innocent, harmless, and defenseless beings, like Boo Radley.
At the end of chapter 30, Atticus and Sheriff Tate are discussing who murdered Bob Ewell, and Heck Tate indirectly tells Atticus that Boo Radley killed Bob. However, Sheriff Tate refuses to inform the community about Boo Radley's heroics because the unwanted attention will harm Boo, who is extremely shy and reclusive. When Atticus asks his daughter if she understands Sheriff Tate's reasoning for protecting Boo, she says,
"Well, it’d be sort of like shootin‘ a mockingbird, wouldn’t it?" (280)
Essentially, Scout metaphorically applies her father's lesson by depicting Boo Radley as a symbolic mockingbird. Given the fact that Boo Radley is a shy, vulnerable person, who spreads joy by giving Scout and Jem gifts, his portrayal as a symbolic mockingbird is accurate.