METHOD:

Preload afects how your suspension works. It is to set the amount of sag from your body weight to make the fork perform best for you. Its worth taking a moment to consider such a simple thing

Budget suspension forks like the Rockshox™ Judy TT that many bike manufacturers have fitted to their low and mid range bikes for some time now have an adjustable preload setting.
Normally forks from large suspension fork companies like Rockshox™, Marzocchi™ & Manitou™ have a manual included with them.

In these manuals recommended levels of ‘sag’ (how far travel goes in when just sat on bike). The level of sag will depend on your weight & riding style. On the Judy TT fork for example the knob is easy to use – clockwise to reduce sag & anti-clockwise to increase sag. My Judy fork came with a manual that recommended a level of sag between 13-20mm on my 80mm fork when sat on the bike normally in a stationary position.

With more complex forks, preload is still one of the main settings, and is always the first to be set. Preload is more common on oil sprung forks, as with air forks you purely pump the amount of air in that the manufacturers manual recommends for you set weight

Depending on your bike, fork, and fork travel options levels of sag will differ. Consult your manual, or if you do not have a manual consult a bike shop or the fork companies website.