Fresnel reflection is a phenomenon that happens on almost all reflective surfaces. Parts of the surface that point directly at
you will reflect less than parts of the surface facing towards you under a greater angle.
The amount of the effect is controlled by
the IOR of the material. You can find this
in the refraction parameters. In real life,
the fresnel effect is always linked to this
IOR. In V-Ray however, you can unlink them,
setting a different IOR for reflection as for
refraction. To do this, click the small L next
to the fresnel check box. The fresnel IOR
box will become available.