A trivet is simply a steel mesh rack that sits inside an oven and prevents food from sticking to the base. It's also useful for airflow and convection when cooking bread, damper or cakes. If baking or frying meat, it allows fat to run off and settle in the base of the oven where it won't be in contact with food.
When roasting, some amount of steaming with water is recommended, and a level of water to just under the trivet will steam and bake any roast or poultry to perfection. Trivets are available in 4.5 and 9 quart sizes.