I decided to produce this as an example of use of apertures and aperture priority.
The above images are taken at standard full-stops from f/1.4 to f/16. Inlaid into the images are the f-stops they were at and also the exposure time. As you can see, as the aperture size decreases (larger f/ number) one full stop the exposure time halves to produce a photo of the same exposure.
On modern (D)SLR's it is possible to select third steps giving even greater flexibility. Than shown above, for more information of F-numbers and apperture sizes see [link]