Atticus's words actually speak for itself. It is just another way of explaining his belief that's it's always best to tell the truth and be honest with his children when they come to him for answers. Atticus's response comes after his brother, Jack, tells him that Scout had come to him with a question he decided was best left honestly unanswered.
"... she asked me what a whore-lady was..."
"Did you tell her?"
"No, I told her about Lord Melbourne." (Chapter 9)
Atticus's little lecture to his brother shows a much greater understanding of children than Jack. Atticus recognizes that a truthful answer works best with children, who can more clearly see an "evasion"--a lie--than adults. Atticus proves his point in a later chapter when Scout asks him
... 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?" (Chapter 14)
Atticus's direct answer, in typical legalese, is truthful and to the point--and one which was above Scout's comprehension, but which sufficiently satisfied her curiosity. She quickly went on to another subject, just as Atticus had explained to Jack.