The kingpin runs through the hangar and two bushings, and then directly into the baseplate. Most kingpins are a fat hex-headed bolt, but there are many kinds of kingpins out there. Grind King kingpins, for example, require a special Grind King tool (they are built that way to give you more room to grind - thus the name).
You can loosen your kingpin to loosen up your trucks, or tighten your kingpin to stiffen up the trucks. I recommend starting out with tight trucks when you are a new skater, and loosening them us as you get more comfortable skating.
The name "kingpin" gets used a lot for a lot of things, but usually carries the same meaning - a piece that holds the whole thing together.
Find out more about the kingpin and working on your skateboard trucks in How to Replace Skateboard Bushings.
