Aperture priority mode on a digital single-lens reflex camera determines depth of field.
The aperture setting controls how much your camera lens opens to capture the image you're shooting. It works like the pupil of an eye: the more the pupil dilates, the more light and image info are admitted into the brain for processing.
Photographers measure the size of the aperture in f-stops—for example, f/2, f/4, and so on. Contrary to what you might expect, the larger the number in the f-stop is, the smaller the aperture is. Thus, f/2 denotes a larger lens opening than f/4.