The Food Hall, also called canteen, mess hall or cafeteria, is an area in each prison. Adjacent to the Food Hall is the kitchen, where chefs roam. It is usually guarded by green doors. Sometimes a 'kitchen' job is available for you to do here.
You are required to attend to the Food Hall when it is Breakfast Time, Lunch Time, or Dinner Time. During Breakfast, Lunch, or Dinner Time, NPC Inmates will wait in line to get their tray of food, then they will sit down in an unoccupied seat and eat their meal.
To hold a tray, go up to the stack of trays and press [interact] (note that this only works during mealtimes). If you sit down on a chair with a tray, you will start eating your food. While eating, your stamina will increase/Fatigue will decrease by about 6 points per second, and health will increase about 2 points a second (verify). Holding a new item or leaving the food hall will make the tray vanish, as will getting up from eating.
In the food hall, there is also a cutlery tray. Opening it will reveal a supply of Plastic Forks, Plastic Knives, and Plastic Spoons, which are accessible but weak tools. Opening the cutlery tray will not alarm guards.