Heck Tate, the sheriff of Maycomb, was probably about to give Atticus even more accolades in regards to his shooting ability, something that Atticus wants to downplay (for good reason). An important precursor to these comments is the fact that Atticus has just saved the town from a very dangerous rabid dog (named Tim Johnson). As the dog wanders listlessly closer and the townspeople hide, Heck Tate calls the situation a "one shot job" while he hands the gun to Atticus (95). Both Jem and Scout are surprised. In fact, their surprise turns to shock when Atticus kills the dog in a single, accurate shot to the head. Afterwards, Jem tries to find a way to bring up the subject. It may help us here to review the actual passage:
"Yes sir," said Jem. "Atticus--"
"What's the matter with you, boy, can't you talk?" said Mr. Tate, grinning at Jem. "Didn't you know your daddy's--"
"Hush, Heck," said Atticus, "let's go back to town." (97)
Heck Tate was probably about to say something like, "Didn't you know your daddy's the best shot this town has ever seen?" or "Didn't you know your daddy's known as 'One-Shot Finch' all over the county?" Instead of flaunting this remarkable shooting ability, Atticus (who is always the bigger and braver man) values his principles of peace much more highly. As a result, Atticus does not stress (or even mention) this ability to his children. Marksmanship isn't a skill of which peace-loving Atticus is especially proud. In fact, Atticus most likely wouldn't value any admiration that his skill would bring upon him. Atticus Finch, in this as in everything else, is nothing short of honorable.
The lesser reason for quieting Heck Tate is that there's a bit of belittling in Tate's voice. Jem and Scout didn't know something about their father, something amazing and well-known, that everyone else in the entire town knew. Heck Tate, whether he means to or not, is taking advantage of that fact and finding Jem's revelation mildly humorous. Atticus, in cutting Tate off, saves Jem further embarrassment. Atticus: an amazing man. (And, you know, I finally met a mother just a few weeks ago, . . . the first I had heard of, . . . who named her first born son Atticus. One guess as to why.)