Tom Robinson is a black man accused of raping a white woman. Atticus Finch is defending him.
Tom Robinson is a good man. From his testimony, we can tell that he is kind, empathetic, generous, and careful. Yet unfortunately his kindness gets him into trouble. He tried to help Mayella Ewell whenever he could because he felt sorry for her. She mistook his kindness for affection and tried to get romantic with him. He rebuffed her, but her father saw them and the damage was done. To save herself from his wrath, she said Tom Robinson raped her.
When Atticus explains this to his brother Jack, it is clear that he knows the chances of winning are not good.
The only thing we've got is a black man's word against the Ewells'. The evidence boils down to you-did-I-didn't. The jury couldn't possibly be expected to take Tom Robinson's word against the Ewells'… (ch 9)
Atticus still tries his best though, not just because he was appointed but because he knows Tom is innocent. He is one of the story's "mockingbirds," a victim who does nothing to bring hate upon himself. Unfortunately, Tom Robinson is convicted and then shot when trying to escape prison.