Core plugs are also known as welch plugs, expansion plugs and sealing discs.
Core Plugs are mainly used in vehicles' engines that contain a 'water jacket' with a constant flow or circulation of water, to keep the engine at a constant temperature. These are set in various places around the engine block and cylinder head.
They are also used for Central Heating Boilers, non-water cooled engines, aesthetics, and general capping or plugging of a void or hole.
Learn how to replace core plugs at How a Car Works.