What secret does Atticus reveal in chapter 26?

In Chapter 26 of Harper Lee's "To Kill a Mockingbird," Atticus shares the truth about Mr. Ewell's death to Sheriff Tate, arguing that Bob Ewell fell on his own knife. Tate and Atticus decide to keep the truth hidden to protect Boo Radley from unwanted attention. This secret reveals the lengths to which the characters will go to protect Boo and uphold justice.