The diffuse component of the reflected light corresponds to the fraction which is emitted homogeneously in all possible directions. The amount of diffuse light depends however on the incidence of incoming light, as in a Lambertian model.

However, Lambert doesn't describe the roughness of a material,  considering it as a perfectly planar surface.
In order to simulate Roughness, the Oren-Nayar model represents a surface as a collection of micro-facets. This model accounts for complex phenomena such as masking, shadowing and interreflections between these microscopic Lambertian facets.  Setting Roughness to 0 leads to a simple Lambertian diffuse color. 

Wrap Lighting is also implemented in the shader, which affects the incidence of incoming light, making it tend to 0°. This effects gives a kind of global illumination look by "wrapping" the light around the object. You can thus have a 180° illumination with a single light source.