Short for maître d'hotel, the maitre d' is a majordomo, a head steward, one who oversees the staff in order to make arrangements for your comfort. When you enter a fancy restaurant, the maître d' is most likely the first person you will encounter.
In addition to welcoming guests to the establishment and seeing that their needs are met, the maître d' recruits and trains personnel and works with the chef to develop the menu.