All internal combustion engines have an engine block and one or more cylinder heads. The mating surfaces where the block and head meet are filed flat for a close, precision fit, but no amount of careful machining can make them completely water tight or be able to hold back combustion gases from escaping through the mating surfaces.
To seal the block to the heads, a head gasket is used. The head gasket has many things it is sealing against. First is the combustion pressure on each cylinder. Oil and coolant must be made to easily flow between block and head and it is the duty of the head gasket to keep these fluids from leaking out or into the combustion chamber, or each other as the case may be.
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