This is a great question. It is best to start with the quote:
He sighed, and said rape was carnal knowledge of a female by force and without consent.
“Well if that’s all it is why did Calpurnia dry me up when I asked her what it was?”
What this dialogue shows is two points. First, it shows that Atticus is not the type of father who will back down from difficult questions. So, when Scout wanted to know what rape was, he told her in a way that satisfied her, even if she did not understand what he meant by it. It is interesting to note that a child does not need to understand something to be intellectually satisfied.
Second, Atticus gave an honest answer. If Scout remembers what Atticus said later in life, then Scout would realize that Atticus gave her an answer that would help her as she matured. So, we can say that Atticus had Scout's maturity in mind when he gave this definition.
Atticus' explanation of rape enhances his image as a father because it shows how he is unafraid of talking about any subject with his kids. While many parents may shy away from talking about such a mature subject with their young kids Atticus does not because he knows that they must learn the meaning of rape one day. Although the typical role of a father is to protect his kids, Atticus shows us that protecting does not mean keeping someone oblivious but educating them in a proper way.