Angle of Incidence refers to the path that light travels from a light source, reflecting upon a surface and then traveling into the camera lens.
The angle of incidence directly affects the sylized appearance of light as it is percieved by the view of the camera and ultimately dictates the "look" of each camera set-up with regard to light and the way it falls upon and gives dimension to the subject.
In general descriptions, light can be referred to in a few descriptive styles or categories for this discussion; Flat, Contoured, or Edge.
In Flat lighting, the angle of incidence would be a small angle or a low number meaning the light source is near the camera, not too far off to one side or the other, and not too high or too low. When the light strikes the subject it covers the subject evenly, not creating dramatic shadows as visible from the cameras point-of-view.
In Contoured lighting, the angle of incidence would be a larger angle for example anywhere from 20 degrees to maybe 60 degrees with regard to the light source emitting and then reflecting off of a subject into the camera lens. The increased angle of incidence means that the light will create shadows on the subject and also increase the effect of shape of dimension on the subject.
In Edge lighting, the light source is generally restricted to being positioned behind the subject anywhere from 60 degrees to a full 180 degrees behind. This angle of incidence will make the subject appear to be only lit along the edges and most of the frontal area of the subject will appear to be in shadow similar to a silhouette.