It is the hitch point of a 5th wheel trailer. The king pin is mounted in the centre of the trailer apron. When the trailer is attached to the truck, the trailer apron rests on the top of the 5th wheel hitch plate.
The different paths that the front and rear wheels take when cornering. The rear or trailer wheels will take a shorter path around the curve, corner or turn. Thus the driver has to compensate for this shortcoming by taking the curve, corner or turn wider.
This piece of the hitch assembly is removable. It is the piece of the hitch assembly that has the ball attached, and can be removed from the rear of the vehicle to prevent banging your shins. When inserting the receiver, it is important to have the locking cotter pin on the pin to ensure that the receiver does not come loose during towing.