Compression is how much the cylinder can pump up; a good test of rings. Leakdown , which requires a special tool, measures the cylinder's ability to hold the pressure. A good test of all the components. As Larry said, if you hear air coming from the intake, then the intake valve is leaking. If it comes through the crankcase, then the rings are probably worn. You get the picture. From what I have learned on leakdown, 25% "total" is considered high, with 35% being maximum. Just my dime's worth and probably subject to "expert" rebuttal.