Shutter priority is semi-automatic. You pick the shutter speed, the camera picks the appropriate aperture setting for a proper exposure. Shutter priority is useful if you find your photos have motion blur after shooting in low light. Often cameras set shutter speeds too low in full-auto mode. To remedy this, switch to shutter priority mode and set the shutter speed at about 1/100 minimum (the smaller the fraction, the less likely it will be that photos are blurry). Bump the ISO up so you're pics aren't underexposed.