This integral written out is words says the total amount of reflected light is the integral over the hemisphere of the product of 1. how much light from wi gets reflected to wr, and 2. the amount of energy hitting the surface.

This integral written out is words says the total amount of reflected light is the integral over the hemisphere of the product of 1. how much light from wi gets reflected to wr, and 2. the amount of energy hitting the surface.