It is a method of molding plastic. The same way many boats are made. When you roto a part like a seat, pillar, step wall, etc. it gives you a more 3D (I guess that is how I would describe it) part. Instead of just being one thickness of plastic it is hollow with multiple sides. It makes for a much stronger part. As an example look at a seat in a Nomad or Jeffe (LL has a nice creek seat as well) and notice how thick it is and then look at a typical play boat seat. They are thin and more flexible. The thicker "3D" seat will take a bigger hit and be stronger under stress. The process is done by opening a mold, putting in a powder like material (the plastic), close the mold, put it in an oven, as the plastic heats up the mold starts spinning so it sticks to the outer wall of the mold. When it is done you open the hold, pop it out and let it cool. Roto is sort for rotational molding.