What does Scout feel sticking out of Bob’s ribs?

After his public humiliation on the witness stand, Bob Ewell has vowed to take revenge on Atticus. One night, he makes good on that threat. Only Bob is such a coward, he takes out his anger and frustration on the Finch children instead. So he attacks Jem and Scout as they're making their way home from the school's Halloween festival. In the ensuing melee, Scout feels someone pull her attacker away. She assumes it's Jem and calls out to him, but he doesn't respond.
Lying on the ground, her movements restricted by her ham costume, Scout reaches out towards the sound of heavy breathing nearby. She thinks it might be Jem, but once she feels the man's stubble, she realizes it's not her brother after all. Furthermore, the man has a knife sticking out of his ribs. The man, of course, is none other than Bob Ewell, and the knife in his ribs was put there by Boo Radley.

On the way home from the Halloween festival at school, Jem and Scout are attacked, and Jem, trying to save Scout, has his arm broken. Scout, knocked to the ground and unable to see due to her costume, starts feeling around for Jem but instead unknowingly comes across Bob Ewell, who has a knife sticking out of his ribs:

Mr. Tate found his neck and rubbed it. “Bob Ewell’s lyin‘ on the ground under that tree down yonder with a kitchen knife stuck up under his ribs. He’s dead, Mr. Finch."

While Atticus believes Jem managed to stab Ewell, Sheriff Tate realizes that it was in fact Boo Radley who stabbed and killed Ewell. However, Tate has no desire to arrest or prosecute Boo. To Tate, Ewell has met his deserved fate: "Let the dead bury the dead." Ewell was responsible for the unnecessary death of Tom Robinson, and now, in Tate's eyes, he had met his karmic end.