予約 means "reservation," "appointment," or "booking." It's the act of arranging to have something, like a table at a restaurant, a hotel room, or a doctor's visit, set aside for you in advance. Example: "レストランを予約する" means "to make a reservation at a restaurant."

ホテルやお店などを前もって取っておくことです。「席を予約する」のように使います。