A maitre d'hotel is someone whose job involves greeting restaurant customers and supervising the wait staff. In a fancy restaurant, the maitre d'hotel is the person who shows you to your table.
Not all restaurants have a maitre d'hotel (or maitre d' for short), but most formal or pricey ones do. You could also call this person the host. Maitre d'hotel originally meant "head domestic" in English — the servant in charge of a home's staff — from the French meaning, "house master." By 1890, the term came to describe the manager of a dining room or hotel.