The document discusses the exposure triangle of aperture, shutter speed, and ISO, which are the three key factors that determine the amount of light in a photograph. Aperture refers to the size of the opening in the camera lens and controls depth of field. Shutter speed refers to how fast or slow the camera shutter opens and controls motion blur. ISO refers to the camera sensor's sensitivity to light and impacts image noise. To achieve a properly exposed photo, the photographer must balance these three elements of the exposure triangle.