In a meeting, it's a customary to bow. These bows vary in an effort, but if one is standing on a tatami floor, it is expected for him to get down on his knees while performing the bow. The longer and deeper bow are the more respect it conveys to the other person. People who hold higher social statuses expect others to greet them with long bows.  Not doing so can result in social snafus.  By the same logic, a smaller nod of the head is used for more casual or informal meetings