What is the "preload" setting on the front fork suspension?
This fork setting exists so that the fork can be customized to your weight (major reason) and riding style (minor reason). It's simply the initial compression of the internal spring in the fork. The more it's compressed, the stiffer the fork will feel.

Bigger preload compresses the spring more, and so it's best for heavier riders and/or people who need/prefer the stiffer ride (racers etc.)  Lighter riders should use less(-) preload. Less(-) preload will also give you plusher ride, but the bike will e.g. dive more during braking.

Source: http://bicyclethailand.com/setting-the-sag-on-mtb-suspension-forks/