Please refer to the link below as this question has already been asked and answered.
As Atticus says to Scout, mockingbirds don't make any trouble for anyone. They don't eat people's gardens, they don't nest in people's corncribs. All they do, Atticus says, is make music for people to enjoy. This can be interpreted to mean that people are mockingbirds when they don't do any harm, they just help other people. In other words, they don't take from anyone, they just give. There are actually two mockingbirds in To Kill A Mockingbird: Tom Robinson and Boo Radley. Tom Robinson was nothing but nice to Mayella Ewell; he helped her around the house and he cared for her in neighborly way, even. And he paid for his generosity and good heart by being killed. As Atticus said "Tom Robinson was a dead man the second Mayella opened her mouth and screamed." Boo Radley, also, did nothing but be kind. He not only saved Jem and Scout's life, but he also gave them gifts; he was, to put it simply, a good neighbor. But he was more than that, he was the children's savior from evil. He was a hero. Boo Radley also did no one any harm--he just made music. And this is why Scout (in one of the most brilliant--if not the most brilliant--scenes of all literature) says to Atticus about telling the town that Boo killed Bob Ewell (justifiably, nonetheless): "it would sort of be like shootin' a mockingbird, wouldn't it?" Scout is 100% correct. Exposing Boo to the gossipy, noisiness of the Maycomb citizens would be like killing him. And so Boo is allowed to "live." And this is a hugely important part of the book; as Scout walks Boo home at the end, she sees, from his point of view on the Radley house porch, the entire events of her, Jem, and Atticus' lives for the past few years. Boo, she realizes, has seen it all. And he closes the door and she never sees him again. I hope this helps. I'm not a teacher or anything (I'm only in tenth grade) but this is my favorite book and I've read it more than 10 times, etc. so I hope that my judgement is good.