The aperture ring is a collar on the lens that can be rotated to increase or decrease the opening of the aperture petals. These opening settings are marked on the aperture ring as "f-stop" values. The smaller the f-stop value, the larger the opening and the more light reaching the camera's detector. See the examples of different aperture settings in the image below.
Examples of different aperture settings
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.
Location of aperture and focus rings on the Iris M Lenses
6mm (Blue) lens
12.5mm (Green) lens
25mm (Yellow) lens
50mm (Orange) lens
100mm (Red) lens
Location of aperture and focus rings on the Iris MX Lens
6mm (Purple) lens
Location of aperture and focus rings on the Iris M Traveler Manual Zoom Lens