The exposure on a photo is how light or dark the photo will appear. The aperture, shutter speed and ISO all change and determine the exposure of the image. Aperture controls which light can enter, shutter speed controls how long light will be entered, for example, the slower the shutter speed, the more light comes in, and the ISO, the ISO controls the sensitivity of your cameras sensor to a given amount of light. Each setting on them controls something different so you have to work with them all to create the perfect picture.