Scout is describing her feelings of her father when she was a child, before she realizes that he is capable of shooting a rifle with more accuracy than anyone else in town.
Scout considers her father feeble because he can’t play football like other kids’ fathers. He is older than most parents of kids their age, and they are disappointed in him because he seems to prefer reading to activity. When Atticus is asked by Heck Tate to shoot the rabid dog Tim Johnson, he takes the dangerous animal down in one shot. This impresses Scout and Jem incredibly, because they did not know he had it in him.
In a fog, Jem and I watched our father take the gun and walk out into the middle of the street. He walked quickly, but I thought he moved like an underwater swimmer: time had slowed to a nauseating crawl. (ch 10)
As an adult, she can appreciate that her father was incredibly talented with a gun. As a child, she did not understand what he was capable of until she saw him shoot the dog.