What does the expression "be good" mean at end of a conversation ?

-It's just a way of saying "good-bye." Based on thing many mothers say to their children as the last thing when the children walk out the door. It has been adapted to a generalization just as asking people "how are you" upon meeting doesn't really call for a litany of their ailments. It is a leftover from the days when people rearely got to see each other & did spend the time catching up.