The MZ series fork is not very good. The only difference between the "comp" and the "race" is the steer tube, which makes it a little lighter.
Preload adjust helps you set the sag on a fork.
Sag is how much travel the fork compresses when you are standing on your bike.
Rebound damping is using oil being pushed through valves to control the speed at which the fork springs back after it is compressed.
On the MZ series, the only adjustment is preload, and when I had that fork, I didnt notice any difference when I tried adjusting it.