Boo Radley had killed Bob Ewell in order to save the children. But Mr Tate didnt wanted it to be exposed it to public. Boo always lived secluded in his house, and rarely came out. And Boo wouldnt like exposing himself ( even in positive way ).
So Mr Tate says that one man ( Boo ) who had saved Arthurs children, it would be a sin to drag him to limelight with his shy ways. And in order to save Boo, Tate concludes that Bob fell on knife himself.
Atticus also understands this, although no one mentiones it explicitly. And so he thanks Boo Radley.