The aperture ring is a collar on the camera that can be rotated to increase or decrease the aperture. The effect is letting more or less light in, respectively. The convenient part about aperture settings is that the amount of light taken into the camera at a particular setting will be equal across different lenses. If you find a good setting for your configuration and decide you need to change lenses, you can use the same f-stop with the new lens.
Examples of the Aperture Ring location on a lens.
Different aperture settings