What is the truth of the stabbing?
Atticus assumes that Jem somehow got hold of Bob Ewell's knife and used it against him to defend himself and Scout from this attach. Atticus is so confident in this interpretation of events and he doesn't want to "cover-up" Jem's actions. His code of honor for the truth wouldn't allow for that. Heck Tate then has to clarify what actually happened -- Boo Radley came out of his house and stabbed Bob Ewell with a kitchen knife. It is this action that needs a "cover-up" with the story that Bob fell on his own knife. Heck Tate suggests that it would be completely unfair to put Boo through the public spectacle that would follow a public announcement of what actually happened the night of the pageant. This action connects to the mockinbird symbolism of the novel. It is a sin to kill a mockingbird that only sings for good -- just as it would be a sin to hurt Boo for only trying to protect the children. The reader has been givin hints that Boo is a misunderstood man who is actually more gentle than the rumors would suggest (ie. the gifts left in the tree) and these events solidify our understanding of Boo Radley.