Light Equals Photography
Light plays a powerful role in photography. In addition, it is relative. A scene/subject will often have varying degrees of illumination. These are commonly know as highlights and shadows. Both highlights and shadows can be a creative tool.
Your job as a photographer is to understand the it, evaluate it creatively and control how the camera captures it. Wildlife photographer Taylor Glenn does a great job of controlling how his wildlife portraits are lit. He often photographs captive animals in studio where he can plan and control the scene. To learn more.