On the front of a 5th wheel trailer is what is known as a pin box (think trailer tongue). It contains the king pin which latches into the jaws of the 5th wheel hitch attaching the truck to the trailer. The hitch is the part that bolts into the bed of the truck and weights about 200# depending upon model.
How a 5th wheel hooks up
https://www.youtube.com/watch?v=7TjF5HrWRAIThe pin weight is the weight of the trailer resting on the 5th wheel hitch or "tongue weight" of a 5th wheel trailer. If the pin weight is too light the trailer will rock back and forth, front to back or "porpoise" as it goes down the road. With 5th wheel trailers most people will exceed the pin weight capacity of their truck before their towing limit.
Where tongue weight is normally 10-15% on a travel trailer, weight a 5th whee'ls tongue or pin weight is 20-25%.
On a side note there are many styles of hitches and pin boxes. Some with air bags for a better ride and designed to slide to allow a sharper turn with a short bed truck.
On my trailer I changed the pin box out for the "Reese Goose Box" which allows me to use a gooseneck style hitch in the bed of my truck (like a horse trailer). I have a" B&W turn over ball hitch" in my truck so I can turn the ball over in my bed and I will have a "clean" bed available to carry things when not pulling the trailer. The hitch mounts below the floor of the bed so only the ball and chain mounts stick through the bed of the truck. Reese Goose Box
https://www.youtube.com/watch?v=AYPzd-KFN3U B&W turn over ball hitch
http://www.sdtrucksprings.com/turnover-ball-gooseneck-hitch?gclid=CNWhiJOXr7kCFdAWMgodmHMAgAOn my trailer, my pin weight is around 2,700# or 22.5% of the trailer. My trailer weighs around 12,000#. Every spring I reweigh it when I've loaded all our stuff for the season.
Hope that clears it up.