A gasket in general is a seal that fills the space between two surfaces. In automotive terms the head gasket is a seal that sits on top of the engine block and underneath a cylinder head to keep gases and liquids from entering into this area as well as preventing external or internal leakage.
Although it’s a very small component of the overall engine, the head gasket plays a very important purpose in sealing the engine’s combustion chamber preventing leakage of the engine’s oil and coolant between the head and block. This in turn ensures the engine’s cylinders have their maximum compression and prevents this pressure from escaping. If the fluids start to mix it can cause engine failure, blue smoke from the exhaust pipe or white smoke form the exhaust pipe indicating a problem.
On older automobiles you will find a head gasket make of composite material or graphite. Newer automobiles use multiple layers steel or MLS for short. They have three layers of steel that are then coated with a material similar to the properties of rubber. This thin piece of metal is one of the main components keeping the engine running temperature optimal.