@ttarchala's answer is awesome. But I thought I could provide a bit more info for anyone else looking for info.
On a coil fork (fork with a spring inside), there are two ways to adjust preload. First off you should get a spring that is the right tension for your weight. For Rockshox forks there are 6 or 7 different spring tensions that are designed for different weight ranges.
See: Rockshox spring diagram.
Once you have the right spring in your fork you can adjust using the preload control (pictured in the question). See @ttarchala's answer for a good description of what this does.
Air forks generally just have one way to adjust fork preload. Looks like this:
(That is a picture of an airpump adding air to an air fork. For more info see: http://bicyclethailand.com/setting-the-sag-on-mtb-suspension-forks/)