The honest assertion byTom that he helped Mayella out with her chores because he ‘felt sorry’ for her is not received well. Scout recounts the atmosphere after Mr Gilmer reiterates Tom’s statement –
…the damage was done. Below us, nobody liked Tom Robinson’s answer.
When Scout refers to the people below her, she means the white observers and the jury. She was sitting in the gallery with the friends of Tom Robinson.
There is no real discernable physical reaction: no-one calls out, or protests, but the atmosphere in the courtroom changes. The tension does physically affect Dill, who has to leave the courtroom as he feels sick -
For some reason Dill had started crying and couldn’t stop.
Tom’s innocent comments are seen as patronizing and insulting. If the jury did have some understanding of Tom’s good intentions, this statement makes him appear arrogant. Suggesting an authority over a white person, even poor white trash like Mayella Ewell, was seen as supercilious and prideful: something a black man at this time could not be.