In To Kill a Mockingbird, what lesson is Calpurnia imparting to Scout when she tells her that Walter Cunningham is her company?Calpurnia takes...

Calpurnia is trying to teach Scout to behave honorably, no matter what the circumstances.  At the foundation of this lesson is that a person can only control how he or she acts, and that maintaining personal honor and dignity are not dependent on the way others go about their business. 

In the case at hand, Calpurnia is speaking to Scout about the virtue of hospitality.  Scout has been raised according to a certain standard, and when people such as the Cunninghams are her guests, she is responsible for treating them in a certain way, making them feel welcome and affording them an attitude of politeness and respect.  How they respond in return is immaterial; the Cunninghams are poor, and for whatever reason may not live up to the same standard of behavior that is expected of Scout.  Walter, specifically, seldom has enough to eat, and pours syrup on his food in a way that, to Scout, is crude and unmannerly.  Calpurnia is trying to teach Scout that it is not her place to pass judgement on others, no matter how they act, especially in her own home.  Even if Walter Cunningham should display such bad manners as "eating the whole tablecloth", it is still Scout's responsibility to be polite and welcoming, and keep her censure to herself.  In this way, Scout is showing herself to be a person of dignity and integrity, because in the final analysis, all she can control are her own actions.

It is significant that this lesson comes from Calpurnia, because, as a black woman in the South, she herself has to put up with the rudeness and abuse of white people constantly, every day.  In the social climate of the times in Maycomb, black people were looked upon and treated as a lower class by virtue of the color of their skin, and forced to evidence an attitude of subservience towards whites at every turn.  Calpurnia understands the conditions in the society in which she lives, and does what she needs to in order to survive, but she acts at all times with a sense of dignity which makes her untouchable in spirit.  She holds herself to a strict standard of behavior, and never allows herself to sink to the level of those around her who do not behave to her standards.  She is a formidable woman, and at the end of the day, nothing anyone can do can take away her honor and dignity, not to mention her sense of kindness and decency.