A four cylinder
internal combustion engine has four
combustion chambers. Similar to how a six cylinder engine has, you got it, six combustion chambers.
An n-cylinder engine has all it's n pistons (one from each of it's n combustion chambers) connected to a single crankshaft. Each cylinder has a combustion, or explosion, once per crankshaft revolution; and the combustions, or explosions, in the n cylinders are equally spaced in time. This allows for a smooth delivery of power.