Exposure is the amount of light collected by the sensor in your camera during a single shot or photograph. If the shot is exposed for too long a time the photograph will be washed out. If the shot is exposed for too short a time the photograph will appear too dark. Almost all cameras today have light meters which measure the light in the given composition and set an ideal exposure automatically. Most people depend on the light meter which is fine, but if you know how to control your exposures you can get some creative and sometimes greatly improved pictures.