A hotelier is a person who runs or owns a hotel. If you stay at a hotel, you may never see the hotelier, who is responsible for hiring and managing staff and keeping things running smoothly.
It's probably more common to use the term "hotel manager," but hotelier is a fancy way to refer to the person in charge of a hotel's operation. If you've got a complaint about your room, you might angrily demand to speak to the hotelier immediately. The word hotelier comes from the French hôtelier, "hotelkeeper or hotel proprietor," and its Old French root hostel, "a lodging."