This is another of the lessons learned by Jem and Scout in To Kill a Mockingbird. Miss Maudie teaches them, through the actions of Atticus, about humility. Atticus had never told his children about his marksmanship skills with a rifle because he was ashamed of them. This skill had an underlying connection with death and killing associated with it, and Atticus probably felt that it was better for his children not to know about his old nickname. Atticus probably felt that his shooting skills were in part God-given, and he (and Maudie) believed that no one should brag about such an inherited talent.
Miss Maudie says these words after Atticus puts down the rabid dog. Scout and Jem are shocked that their father is the best shot in town. They also learn of his nickname - "One shot Finch." The children wonder why they never knew of Atticus's ability to shoot.
Miss Maudie gives them the answer. Atticus is a modest and humble man. Moreover, good people do not boast. Finally, the ability to shot, according to Miss Maudie is different than other talents. Since shooting involves death, it should only be used when there is a necessity. Up to this point, there was no necessity. This is part of the reason why Scout and Jem did not know about their father's ability to shoot.
“Maybe I can tell you,” said Miss Maudie. “If your father’s anything, he’s civilized in his heart. Marksmanship’s a gift of God, a talent—oh, you have to practice to make it perfect, but shootin’s different from playing the piano or the like. I think maybe he put his gun down when he realized that God had given him an unfair advantage over most living things. I guess he decided he wouldn’t shoot till he had to, and he had to today.”
“Looks like he’d be proud of it,” I said.
“People in their right minds never take pride in their talents,” said Miss Maudie.