A Mason jar is a molded glass jar used in canning to preserve food. The mouth of the jar has screw threads on its outer perimeter to accept a metal ring (or "band"). The band, when screwed down, presses a separate stamped steel disc-shaped lid against the rim of the jar. An integral rubber ring on the underside of the lid creates a hermetic seal to the jar. The bands and lids usually come with new jars, and bands and lids are also sold separately; while the bands are reusable, the lids are intended for single use when canning.
While largely supplimented by other methods for commercial mass production, they are still commonly used in home canning.