In photography, exposure is the amount of light per unit area (the image plane illuminance times the exposure time) reaching a photographic film or electronic image sensor, as determined by shutter speed, lens aperture and scene luminance. Exposure is controlled by three functions. Shutter speed, which determines the length of time you allow light into a camera. Aperture, which determines the size of the opening through which light enters the camera. ISO, which corresponds to the sensitivity of a digital cameras image sensor to light.
These two pictures above are my worst images there are not perfectly exposed the one on the left is under exposed and the one on the right is over exposed.
This picture is my best picture it is perfectly exposed.It has great colours and the mainparts of the image are easy to see.It doesn't have to much light, it is just right.