Ok dumb question, but I can't seem to find this anywhere. What exactly is a compression check and the numbers. What is it mesuring. I'm pretty sure it's something about how much your cylinders leak... but where to? I only have a vague idea, can someone clear this up for me. Thx!

A compression check basically measures if your cylinders can hold pressure properly. If there is something wrong with the pistons or cylinder walls then the numbers will come back lower than normal. This is bad since your car will make a lot less power.