halfbellstateResin in general, particularly as regards things like fountain pens is typically more akin to something like an acrylic or epoxy, if you are familiar with those. Typically, the material will be consistent throughout, unlike many plastics which will often have a surface colour over a different interior. Precious resin is essentially this same kind of material, except with the addition of other particulates to change or improve the textured feel, appearance or other qualities of the resin.
It is marketing speak for expensive plastic, but it is also a better material than most plastics you encounter on a regular basis.