What is an F-Stop in Photography?

it decideds why the iris of the eye opens and closes, it decides how much light is let into the picture. The higher the number, the less light let into the picture. the lower the number the more light. It also it decides how much it is in focus. So a higher number is a hight depth of field, meaning alot in focus, and a lower number is a shallow depth of field resulting in a blurrded back ground and a focused subject.