In Chapter 9 of To Kill a Mockingbird, Uncle Jack and Atticus discuss the upcoming trial of Tom Robinson as well as dealing with children. Atticus tells his brother to answer a child when that child asks a question. In his speaking while he knows Scout is listening, Atticus is surreptitiously doing just that: answering his child's questions about the trial. He hopes that she will be more attentive, believing that she is privy to something she should not be. In this way, Atticus believes his words will make more of an impression and Scout will act on her own to behave as Atticus wishes by "keeping her head" and by understanding that he must defend Tom because he cannot shirk his responsibilities.
When Scout narrates, "... it was not until many years later that I realized he wanted me to her every word he said," she indicates that she has learned the lesson that Atticus imparted in his talk with his brother. It has been a lesson of integrity and of tolerance.