This section of the novel comes in Chapter 14. Earlier, in Chapter 12, when Scout and Jem are walking home from church with Calpurnia, Scout asks Calpurnia why people won't hire Helen. The answer to this is Scout's introduction to the word "rapin'." When Scout asks Calpurnia what it means, she is told to ask "Mr. Finch." Then, in chapter 14, after a white man has walked past her muttering "They c'n go loose and rape up the countryside for all of 'em who run this county care," Scout is reminded that she has not ask Atticus the meaning yet.
The decision of Atticus to defend Tom Robinson against the charge of rape is one that is not gaining him popularity amongst the white residents of Maycomb, and Jem and Scout are beginning to realise that their father's decision is impacting them. However, Scout is still unclear about what "rape" actually means. Note how Atticus responds to her question:
He sighed, and said rape was carnal knowledge of a female by force and without consent.
Key to focus on is how Atticus sighs before responding. This indicates some sadness on his part as he reflects that the adult world of messy, ugly realities is breaking into his daughter's childlike innocence. It seems clear that he gives a technical, legal definition precisely because he is trying to do his best to keep the exact ugly truth of the nature of rape from his daughter for as long as possible. Whether he is right to make this decision is a moot point, as the novel narrates Scout's move away from childhood and her coming of age as she learns more about the adult world, and it is clear that she will have to learn the precise definition of rape eventually. However, at this point in the novel, the reader witnesses the attempt of Atticus to shield her as much as possible from truths that would shatter her innocence.