leakdown test?

i was wondering what a leakdown test does exactly and how you perform it

it will tell you if your cpmpression is ok and can help tell if your valves are leaking by or rings you can do a wet and dry leakdown test you need guage and disconnect coil wire thread the guage hose in spark plug hole and crank the engine ...then go write the numbers down..thats the dry test after your done with all four cylinders ...squirt a lil oil in the cylinders and do it again thats a wet test......dunno if it helps