The kingpin (aka swivel pin) is the pivot in the steering mechanism of a vehicle. This component takes a fair amount of stress and can wear out quickly if not regularly greased.
This post goes over how I grease the kingpins on my LDV Convoy van, but the following method applies to pretty much all vehicles with kingpins. This is a simple and quick job (15 minutes max) which will help prolong the life of the kingpins. I do this job once a month as advised by a reader of this blog, who also has the same LDV van.