How does Scout react when Atticus explains to her why he has decided to defend Tom Robinson?
I know Atticus explains he does it because he wouldn't respect himself if he didn't, but I dont understand how Scout reacts.
Scout is young and doesn't understand everything her father is telling her--yet. In fact, Harper Lee often uses Scout's youthful ignorance as an opportunity for the adults to explain what she wants the readers to hear about prejudice and hate and other harmful stereotypes.
In this case, Atticus does explain his position, as you noted. He tells Scout, in answer to her question, that they will probably not win this case. He then asks her for one thing--to keep her cool and not pick a fight over this matter. Her reaction is pretty basic, considering she really doesn't understand what her father is asking her to do--to accept that others will be ignorant and cruel and hurtful because they have not learned better. She is capable of being the bigger person, and that's what he asks of her. She promises.
She tries, she really does. but when her cousin Francis calls her father names she simply has to haul off and whack him--prompting Atticus to let Scout overhear a conversation in which he admits his fears for the summer ahead and his hope that his children will come to him first rather than try to fight. She doesn't make another promise, but she does better the next time she is tempted.
I mean DEFINE*