I assume that you are referring to the advice given by Miss Maudie to Scout concerning her father in Chapter 10 of the Harper Lee novel, To Kill a Mockingbird.
The chapter begins with Scout lamenting about Atticus's limitations.
Atticus was feeble; he was nearly fifty... He couldn't do anything.
Atticus refused to teach Scout and Jem how to shoot their air rifles--presumably because Atticus could not shoot--but instead warned them that "it's a sin to kill a mockingbird." Miss Maudie explained to Scout why this advice was so important and then launched into a her defense of Atticus's hidden talents. She explains that "he can make somebody's will so airtight can't nobody meddle with it." He is the best checker player in town, and he can play the Jew's Harp.
Later that day, Atticus would surprise Jem and Scout with his true marksmanship skills when he kills the mad dog, Tim Johnson, with a single shot above his left eye. Sheriff Tate offered his advice.
"You were a little to the right, Mr. Finch," he called.
"Always was," answered Atticus.
"One-Shot" Finch, the best shot in the county, had returned to action after a 30 year absence. For Jem and Scout, it was yet another example of the many hidden talents of the ever modest Atticus Finch.
Scout believes that all people are inherently good. Because of her youth and innocence, Scout cannot see the evil that resides in human nature. Miss Maudie says in chapter 5, "There are just some kind of men-who who're so busy worrying about the next world they've never learned to live in this one, and you can look down the street and see the results." Miss Maudie knows that there is evil in the world. However, Scout must go through her own trials to realize this.
Since Scout is a young innocent child she believes that everyone is good and she does not want to believe all the bad and evil in the world. However, Miss Maudie is an adult and she realizes the rudeness and racism found within the world, but Scout has to go through this alone. This is a journey for her as the book progresses.