In terms of 3D software this is the angle of the face normal compared to the camera facing. Note the principal BDSF has this already build into it. So for example if you have a sphere and the camera is looking directly at its center. The center of the image would be 1. As you move to the outer edges of the sphere it will move towards 0. In the real world, beaches and lakes are the best examples.(it can also be seen in puddles) when you look straight down on the water you can see through it, but as you view it at an angle it becomes more reflective or mirror like. Hope that helps
