This conversation occurs after Jem has been attacked on his way home from a school play. Atticus Finch, Jem's father, and Heck Tate, the sheriff of Maycomb, are arguing about what actually happened during the attack on Jem and his sister, Scout.
Scout recalls Jem jumping on top of Mr. Ewell, pulling him off of his young sister, and supposedly stabbing him. The sheriff on the other hand, asserts that it was Mr. Ewell who fell on his own knife during the struggle.
Atticus, an attorney, believes that Tate is just saving his son from the reality of his actions and wants all the information out in the open. He does not want Jem to have to live under this shadow of rumor and suspicion his whole life. This is ironic because most parents would and do jump at the opportunity to save their children from harm even if they are truly at fault.
Tate, the sheriff, stubbornly asserts that Mr. Ewell fell on his knife. Later the reader learns that he, too, believes Jem stabbed the man, but he does not want attention drawn to the actions of such a vile man. Tate explains this by noting that Mr. Ewell was responsible for the suicide of accused rapist Tom Robinson:
There’s a black boy dead for no reason, and the man responsible for it’s dead. Let the dead bury the dead this time, Mr. Finch. Let the dead bury the dead (Ch. 30).
Typically, a sheriff would push for each detail to be public and presented in court. Ironically, Tate chooses to avoid putting any more attention on the evil Bob Ewell, calling that "a sin."