Cecil Jacobs insulted Atticus by saying he defends “niggers.”
Atticus has told Scout not to fight, but she has to fight Cecil Jacobs.
Cecil Jacobs made me forget. He had announced in the schoolyard the day before that Scout Finch's daddy defended niggers. I denied it, but told Jem. (ch 9)
Scout has no idea what he means by this. When people call her father a “nigger lover” she knows it is an insult, but does not know what the insult means. She even asks Atticus, who tells her that he is just defending an nice Negro named Tom Robinson, and he is just doing his job. He tells her not to use the word "nigger" because it's common. He does not want his children to grow up racist.
The fight with Cecil is just one of many Scout will have as the people in the town and in her own family try to wrap their heads around the fact that Atticus actually is trying to defend Tom Robinson. The fact that the man is innocent never seems to enter anyone's mind, and even Atticus knows there is no way he can win. He still wants to try though, to prove a point- all men are created equal.