Aperture Priority (Aperture Value on some cameras) like shutter-priority, allows the photographer to take control, but the objective is different.

A shutter-speed and an aperture give a correct exposure, but apertures also control depth-of-field. This is explained elsewhere, but a small aperture (f22) creates an extended depth-of-field, whereas a large aperture (f2.8) creates a shallow depth-of-field. The values in between extend or decease the available depth-of-field, but the exact amount depends on several other factors including focal length of lens and physical size of sensor.

Aperture Priority is useful for shooting into the light, particularly if the sun is in the composition. This scenario should come with a health warning regarding your eyesight, it can also cause degradation to the image known as flare.

Flare is avoided by closing down the lens to its smallest aperture - f22 on DSLR cameras, f8 on most compacts and bridge. There should be sufficient light to handhold, but using the smallest area of the lens reduces flare.