In chapter 3, Atticus invites Walter Cunningham to stay for dinner. We have previously met Walter in Scout's first-grade classroom when he shows up without a lunch but won't take a quarter from the teacher to buy one. This refusal establishes the Cunninghams as part of the respectable poor in Maycomb who won't take a handout.
At dinner, Scout is horrified and critical when Walter pours molasses all over his meal. She has learned enough about middle-class habits to know this behavior is odd. Calpurnia, however, pulls Scout into the kitchen angrily to tell her to never be so impolite as to criticize another person's eating habits, especially if they are of a lower class than you: hospitality means putting other people at their ease, not making them feel uncomfortable.
This episode has a comic quality, but it functions on multiple levels to educate the reader about Maycomb. First, it continues to establish the Cunninghams as the worthy poor in the class hierarchy, which will later become important when the focus shifts to the audience needing to understand why the Ewells are considered white trash.
Further, Lee is showing that color and true class don't necessarily go hand in hand. Calpurnia may be Black, but she shows much more genuine class than Aunt Alexandra will later when she refuses even to have Walter to dinner because she feels he is a bad influence on Scout. Lee teaches through this episode that white feelings of superiority on the basis of being white are misplaced.