Before this chapter, Scout and Jem had been a little bit embarrassed that their father didn't do anything "cool" for a living, or wasn't super strong, or didn't have any hobbies that they could brag about to all of their friends. Kids at school occasionally get into the "my dad's cooler than your dad because...." discussions, and when that happened, Scout and Jem didn't feel they had anything to offer. Scout says, at the beginning of the chapter, that
"Atticus was feeble...which we felt reflected upon his abilitites and manliness...and there was nothing Jem or I could say about him when our classmates said, 'My father--'".
They had no idea that Atticus did anything other than read books and be a lawyer--very boring indeed. But in chapter 10, as Atticus reveals, through the shooting of old Tim Johnson, the rabid dog, that he is an excellent marksman, Jem is shocked. He had no idea that his father could shoot so well, and with such seeming ease. It is super cool! He is really excited, but perplexed as to why Atticus wouldn't tell them. Maudie lets him know that Atticus doesn't like to brag, and if people knew he was so good, they'd feel bad about their own abilities. Jem ponders this, and respects it, and even tells Scout to not brag about Atticus at school. If Atticus wants to keep it quiet, they should respect that. Jem realizes that "Atticus is a gentleman," as he says at the end of the chapter. Not only is he cool for having a skill, but even cooler for not bragging about it. He realizes his dad is respectful and gentleman-like. I hope that helps! Good luck.