A spark plug socket is a deep, thin-walled socket used to replace spark plugs. Spark plugs are made of fragile ceramic insulation that can crack easily if hit. They also sit deep inside narrow cylinder head wells where standard sockets cannot reach. The spark plug socket features a rubber boot, foam sleeve, or magnet inside the socket head. This grips the plug's white ceramic body, holding it secure so it doesn't fall out as you lower it down the engine well.
If you drop a spark plug down the well, you can crack the ceramic nose or bend the metal electrode gap, leading to cylinder misfires. The socket is designed with a thin outer wall to clear the tight bore of the aluminum head. When installing plugs, always start the threads by hand using the socket and extension without a wrench. This helps you feel if the threads are cross-threading before you apply torque.