An engine is like an air pump. It needs good compression for it to work efficiently. Most cars should have about 140 to 160 lbs of cranking compression. There should be no more than 10% variation between any of the cylinders.
Some outcomes are, if there is low compression in one cylinder, that usually indicates a bad exhaust valve. Low compression in two cylinders (adjacent to each other) means the head gasket is bad. Low compression in all cylinders means the rings and cylinders are worn and the engine needs a rebuild.