Rations are used by the military, typically in sieges or prolonged warfare. They are made from most foodstuffs, and three military rations can be put together from one item of full satiation food. To make a ration you need to type MAKE RATION FROM followed by the item of food you wish to use.
You can keep your military rationed up by either accompanying them with carts or wagons filled with rations or by equiping them directly with EQUIP <legion> WITH <number of> <ration>, e..g. EQUIP THAKRIANS WITH 50 RATIONS. It is safer to carry your rations around in guarded wagons separate to the legions themselves.