Everything in photography is about light. How much you let in is the key. The less your image is going to be darker and the more it will be brighter. Now over or under exposure of your sesor or film can work to make a great photograph. Just remember photography is a form of art. You don't have to follow the rules.
Light can come from any place, be it the sun, a flash or a lighter and so on. Without the light you can't get that image
