Shutter-priority describes a metering method which automatically sets (or displays) the lens aperture, based on the shutter speed chosen by the user. The term typically refers to a camera offering autoexposure—for example, one where an A or EE position is found on the f-stop ring. But shutter priority could also describe a method of manual exposure metering, i.e., one where the photographer selects the shutter speed first and then chooses the aperture based on the light value and the shutter speed. It is even possible to think of shutter-priority exposure in fully manual contexts with no meter: many manual photographers set their cameras to a known benchmark for a certain brightness level and then adjust the aperture only to adjust for greater or lesser brightness levels.
Shutter Priority may be be the preferred type of metering when the photographer needs to control the amount of motion blur in a photograph. A slow shutter speed will blur moving subjects, whereas a high speed will stop action and reduce the effect of camera shake, rendering a sharper image.
(Compare with Aperture priority.)