Aperture refers to how wide the shutter is allowed to open when a picture is being recorded. It is controlled by steps expressed as f-stops, like this: f/5.6. The size of the hole controls not only how much light is allowed in, but how much of the scene from front to back will be in focus (depth of field). A general rule of thumb is the smaller the number ("opened up"), the larger the opening and the shallower the depth of field. The larger the number ("stopped down"), the smaller the opening and the greater the depth of field. When a photographer shoots "wide open" it means he's using the smallest f-stop his camera is capable of obtaining (and therefore the largest aperture opening). When he refers to "stopping down", it means he is narrowing the size of the aperture.