A buyout is when someone buys out a restaurant for an afternoon and or an evening for a private event. A coordinator works with the client to come up with a menu, determine space arrangements, coordinate flowers, special table settings, and a price is set for the event.
Restaurants love buyouts because they can set a price commensurate with a full night of regular service revenue and will generally come out way better on food cost.
As a front or back of house employee, it's a welcome break from the typically hectic night of service, BUT standards for banquet food are typically much higher so it is wise to come to work expecting to focus as you will be sending out large volumes of food all at once which will all be considered VIP.
For the perspective client, a much cheaper alternative to a buyout is a reservation in a private dining room (PDR). Most upscale restaurants will offer private dining events for groups of 8 to 50 guests with their own service staff, private bathrooms, and currated menus.