Jem and Scout had earlier been quizzing Miss Maudie about their father's lack of skills and abilities.
Atticus was feeble...
Our father didn't do anything.
But when Tim Johnson, the mad dog, came wandering down the street, it was Atticus that Sheriff Heck Tate turned to when it came time to take the dog out. In a move that shocked Jem and Scout, the sheriff "almost threw the rifle at Atticus." Atticus
"... just relaxed all over, an' it looked like that gun was a part of him... an' he did it so quick..."
It was Miss Maudie who told the children about the skill she had forgotten to mention before: Atticus had been the top marksman in Maycomb County when he was a youth. When Scout wondered why Atticus "never goes huntin' now," Miss Maudie explained that Atticus was "civilized in his heart," and his God-given talent was
"... an unfair advantage over most living things. I guess he decided he wouldn't shoot till he had to, and he had to today."
Scout couldn't wait to get to school to tell all her friends about what she had just seen--and about her father, "Ol' One Shot"--but Jem thought differently. He told Scout to keep quiet and that Atticus must have had a good reason not to tell them; that it was his humble nature that kept him from bragging about his talent. Jem understood this positive aspect of his father's personality, telling Scout that
"I wouldn't care if he couldn't do a blessed thing.
"... Atticus is a gentleman, just like me!"