To reheat means to heat something that has already been cooked before, in order to bring it back up to a serving temperature. This can be done using various methods such as microwaving or placing the food in an oven or on a stove top burner for a short period of time. Reheating is often necessary when leftovers are stored and need to be re-warmed before consumption, but it can also be used to revitalize certain dishes that may have lost their heat during the cooking process.