I believe that would give the card the ability to render non-polygon based objects. With primitive rendering objects are described (mathematically) instead of saved as a 3d model, which is a collection of points (vertices).
Edit: just looked it up on Futuremark's vocabulary guide:
Primitive
This is the term used to describe a shape or object in the world of computer graphics. A primitive may simply be a single point or a line, or it could be a triangle or rectangle (quad). Most desktop graphics adapters produce primitives by interpolating vertices into the required shapes - they are then textured one at a time until the whole scene is complete.
Anybody correct me if I am wrong.