Preload= stiffness & Mystery button with R on it= Rebound, which is speed/power the fork pushes back up after compressing.
I had an RST fork with preload and stuff and was wondering why they turned infinitely and didn't do anything. took them off and found they are just caps popped into the tube, they didn't connect to anything, just decoration.
An example of rebound is I have an old bike with no rebound control, if you hit something hard enough to compress the shock all the way down, you have to be prepared for it to come back up, or it will throw you off!
if your Rebound is too high the fork will kick up really hard after a big bump. if it's too low it may take too long to come back up, so in rough choppy sections it could bottom out or have wierd handling.