Simply put, Bob claims that Tom Robinson has raped and beaten his daughter, Mayella. According to Bob's testimony, he returned to his house, looked in the window, and
"--I seen that black nigger yonder ruttin' on my Mayella!"
Much to his pleasure, Bob's statement sends the courtroom spectators into an uproar. Bob claims that Tom not only raped Mayella but also beats and chokes her, causing the bruises on her neck and face. Bob also "holds with Tate"--he agrees with all of Sheriff Tate's testimony, that no doctor was called and that the right side of Mayella's face was bruised. Atticus also requests that Bob write his name, and he did so with his left hand.
Scout and Jem (and most readers) choose to believe Tom's testimony: that Mayella hugged and kissed him while he was trying to get a box off the top of her chiffarobe, and Bob appeared as Tom was trying to run away. Since Atticus proved that Mayella's bruises could only have been inflicted by a left-handed man (Bob writes with his left hand); and since Tom's left arm is crippled, it can be assumed that the only other person present--Bob Ewell--choked and beat his own daughter. Bob must have been angry to see Mayella kissing a black man and, according to Tom, Bob told her,
"... you goddam whore, I'll kill ya."