The aperture setting or F-stop controls the depth of focus of a photograph. This is useful if you want only part of the subject (what ever is closest) to be in focus or if you want the entire picture plane to be in focus (such as a landscape). This also works with the shutter speed and ISO settings.
This is an example of an open aperture setting (such as F5). It lets a lot of light in, but only what’s closest is in focus.
A more closed aperture setting, such as F22, provides for a much deeper depth of field. However, not as much light comes into the lens. The amount of light coming in can be controlled with the shutter speed and ISO setting. Here is an example of a photo with a depth of field:
