Bob Ewell testifies that he heard his daughter Mayella yelling and saw Tom Robinson raping her, then Robinson ran away. This is a blatant lie, of course. Atticus proves that Robinson could not physically have committed the crime and Bob Ewell most likely did it.
"Well, I run around the house to get in, but he run out the front door just ahead of me. I sawed who he was, all right. I was too distracted about Mayella to run after'im. (ch 17, p. 127)
Ewell’s testimony is enough to convince the jury though, because he is white and his daughter is white. It is impossible for a white jury to take the word of a black man (or his lawyer) instead of that of a white man. Atticus knows this, and even though he tries his best, he realizes Tom is going to be convicted.