The Prim is the Prima Materia of Second Life, everything is made of this "stuff", and like alchemy (often called The Art) we can, through diligence, ritual and devotion turn this humble prim into pure gold. "Everything is made by prim and without prim is nothing made that was made." (more)
The Temple of the Prim, Beta v 1.0
The Prim is totally neutral, even more neutral than matter which has imbued feeling depending on whether it is wood, silver, etc. The Prim ONLY manifests its creator's psyche, nothing else.
It is the blankest of blank canvases.
It is, actually, nothing. Matter is actually an illusion too. Atomic physics teaches us that electrons are energy defined by their general relative positions, there's no 'stuff'. Solid is illusion. (more)
Update: See Bettina Tizzy's report on NPIRL (link)