A canning lid is a flat, round piece of metal or plastic used to seal jars during the canning process. It creates an airtight seal that helps preserve food by preventing bacteria and air from entering the jar. Canning lids are typically used with glass jars, which are often made of glass and designed for food storage. These lids come in various sizes to fit different jar openings, and they usually have a rubber gasket on the underside to enhance the seal. After filling a jar with food, the lid is placed on top and secured with a canning ring to ensure a tight fit during processing.