Atticus has just very efficiently shot the rabid dog that had the entire street terrorized. When he came back to find Jem "paralyzed" with awe that his father could shoot and had never told him, Heck Tate says, "What's the matter with you, boy, can't you talk?...Didn't you know your daddy's-" and this is where Atticus tells him to zip it. What he was probably going to say is what Miss Maudie later divulges to the children: that Atticus is and always has been the best shot in town. However, Atticus doesn't want that getting around; he doesn't want to brag about his talent because "people in their right minds never take pride in their talents" and Atticus thought that his talent was "an unfair advantage over most living things." So, he doesn't want Jem to know, because he doesn't want to tout his talent, or have Jem touting it to all of his friends. Atticus doesn't want anyone to feel bad about themselves because of how good he is at something. Jem respects this decision in the end, and is incredibly proud of his dad.