A compression test will determine the health of the upper piston ring by making sure the air inside is being compressed within a certain specification. Your specification should be around 180 psi across all cylinders. A leakdown test compresses the cylinder with air and measures how much air (as a percentage) leaks through either the piston rings, valves, or head gasket. The smaller the leakdown numbers, the better, obviously. These are both tests to ensure the engine is healthy.