Let's see if we can answer this with a little explanation and the interactive aperture demo below.

Before we get started on the details of how to control aperture, it is first important to understand what aperture is. Aperture is defined as, "a hole or an opening through which light is admitted." In your camera lens, as the diaphragm opens and closes, it allows more/less light into your camera, very much in the same way the pupil of your eye opens and closes to control the amount of light allowed in.

The graphic below, on the left, illustrates the inside of a camera lens and how it changes in size as the aperture (also know as f-stop) is adjusted. The lower the number, the wider the aperture is opened and the more light that is allowed in. This also has a direct effect on depth of field (DOF), which is the amount of depth in the scene that is in focus. A low f-stop (f/5.6 or lower) creates a wide DOF.

By sliding the red control from left to right, you can see how the aperture iris contracts, reducing the amount of light and DOF and brings distant areas of the scene into focus.

f/4

(Understand that this is a very simple approach to explaining aperture and there are many other aspects to how aperture controls DOF and that every camera lens has a different f/stop range. For illustration purposes we choose f/4.0 to f/16 but many camera lenses go lower than f/4.0 and much higher than f/16.)