In Chapter 10 of Harper Lee's To Kill a Mockingbird, soon after the Finch children receive the air riffles they had asked for as gifts for Christmas, Atticus shows his general displeasure in the idea of killing living things. He tells the children he would rather see them shoot at tin cans but understands they'll be tempted to "go after birds." He then gives them the advice upon which the title of the book is based. Scout is so surprised by his use of the word "sin" that she asks Miss Maudie what he means. Miss Maudie explains that other birds can be nuisances by eating up gardens and other misdeeds, but mockingbirds "don't do one thing but sing their hearts out for us" (Ch. 10). Through this explanation, Scout is able to understand that it is wrong to harm anyone or anything that is innocent and does nothing but bring pleasure.
Though it takes Scout a while to absorb her father's advice, by the end of the novel, Scout has matured to the point that she can appreciate its full importance and act upon it. By the end of the novel, Scout comes to understand that it was their neighbor Arthur (Boo) Radley who saved Scout's and Jem's lives by attacking and killing Bob Ewell with a kitchen knife from the Radley home. She further understands Sheriff Heck Tate's reasons for insisting Atticus not pursue the issue in court but rather keep things quiet. Sheriff Tate argues that it's not "against the law for a citizen to do his utmost to prevent a crime from being committed"; furthermore, it would be a sin to drag shy Arthur "into the limelight." Scout agrees with all Sheriff Tate argues and says to her father that exposing Arthur would be "sort of like shootin' a mockingbird, wouldn't it?" (Ch. 30). In other words, Scout is asserting that it would be wrong to harm Arthur when all he has done is provide pleasure by caring for the Finch children, which shows just how much she has finally been affected by her father's advice to protect the innocent. She has been affected to the point that the advice now governs her thoughts.
Prior to this moment in the book, Scout struggles greatly with the concept of not harming the innocent. We see her struggle when she aims her air riffle at Miss Maudie's rear end in Chapter 10 and again when she nearly squashes a roly-poly near her cot one summer evening in Chapter 25. In contrast, Jem shows he has internalized the lesson of not harming the innocent when he commands her not to squash the bug and to "set him on the the back steps" instead. Scout's transition from struggling to understand to finally understanding the need to protect the innocent shows just how much she has grown as a character through her encounters with evil in the world and her loss of innocence. It further shows her father's advice to protect the innocent has so deeply affected her that the advice now governs her actions.