FRN is an acronym for Fiberglass Reinforced Nylon. Basically it is a Nylon polymer mixed with glass fibers to add strength to the material that can be produced in a large number of colours. The handles of the knives that feature FRN as material are made by injection molding which allows to produce large quantities easily.
Spyderco uses FRN on its cheaper models but that doesn't mean that the material is inferior. In fact it is a very durable and long lasting material for knife handles.