"There's some folks who don't eat like us," she whispered fiercely, "but you ain't called on to contradict 'em at the table when they don't. That boy's yo' comp'ny and if he wants to eat up the table cloth you let him, you hear?"
"He ain't company, Cal, he's just a Cunningham-"
"Hush your mouth! Don't matter who they are, anybody sets foot in this house's yo' comp'ny, and don't you let me catch you remarkin' on their ways like you was so high and mighty! Yo' folks might be better'n the Cunninghams but it don't count for nothin' the way you're disgracin' 'em—if you can't act fit to eat at the table you can just set here and eat in the kitchen!"
Calpurnia sent me through the swinging door to the diningroom with a stinging smack. (3.26-29)
Cal's moral lesson here is to respect people's differences, even if you think you're better than them. And acting like you're better than other people is the surest way to show that you're not. This interaction is an early blow against the stereotype that white people have morals but African-Americans don't—and Cal follows it up with a loving "blow" of her own. There's nothing like a smack to make a lesson hit home, right?
