You don't need to use it. If you want to use it, you must use a special film for microwave ovens, which is marked with a plastic film with heat and high temperature resistance. Ordinary membrane will release harmful substances at high temperature, so it is best not to use it. Generally, some people use a film to lock moisture when heating to prevent water vapor from evaporating. When heating food, you can cover the bowl with heat-resistant plastic (plastic cover of pp5 microwave oven). If there is no oil that is easy to spill, there is no need to cover it. If the food is too dry, you can pour some boiling water on it and reheat it.