A mockingbird is a bird thought to be innocent because all it does is provide music to be enjoyed by others. Do you believe Boo Radley to be a mockingbird?
Scout, the narrator of the story, believes that Boo is like a mockingbird. Boo brings joy to the children and shows them kindness. Eventually, he saves their lives.
Boo hides gifts for Scout and Jem in the knot-hole of an old tree on the Radley property. These gifts delight Scout and Jem. The siblings eagerly look forward to each gift. When Jem's pants get trapped in the barbed wire at the edge of the Radley yard, Boo mends them. On a cold night, Boo covers Scout with a blanket as she rests outside.
On a walk home from the Halloween festivities, Bob Ewell attacks Scout and Jem. He tries to kill them, but Boo comes to the rescue. He fights Mr. Ewell and stabs him. Heck Tate wants to imply that Mr. Ewell fell on his knife. He wants to spare Boo any hardship or a long trial. Atticus tries to make Scout understand this. She does:
"Yes sir, I understand," I reassured him. "Mr. Tate was right."
Atticus disengaged himself and looked at me. "What do you mean?"
"Well, it’d be sort of like shootin' a mockingbird, wouldn't it?"
Scout sees Boo's innocence. She knows he was only trying to save them. She knows that he means no harm, just like a mockingbird.