The preload adjustment on your fork is to adjust "sag" or how much the fork compresses under the static weight of you and your bike. This is to reduce "topping out", as Kimmo notes. Your fork manual should have a recommended sag setting but, in general, it's around 15-25 percent of your fork's total suspension travel, e.g. 15-25mm on a 100mm fork.

To adjust, wrap a zip tie around one fork leg and, with all weight off of the fork, push the tie down the leg against the fully extended fork lower. Then, sit astride your bike in a normal riding position. Your and the bike's weight on the fork will cause it to compress somewhat and move the zip tie up the fork leg. Get off the bike and measure how far the tie moved up from its unweighted position. That's sag. Turn the preload adjustment knob to increase or decrease sag to match Suntour's recommended settings. Good luck!