Trucks act like the the axle for a skateboard. Trucks are the metal T-shaped part that mounts onto the underside of the skateboard deck, that the wheels will be attached to. A regular skateboard will have two trucks, each facing each other.
Each truck is made up of a hanger, two bushings, two washers and the mounting nut. The axle runs through the hanger, which is attached to the baseplate with the kingpin. The kingpin runs through the bushings and washers. They cushion the hanger as it turns. The tighter the kingpin, the stiffer the turns. A looser kingpin will provide tighter turns, yet less stability.