Shutter Priority
A semi-automatic mode, marked S or Tv, in which the photographer sets shutter speed and ISO while the camera chooses aperture.
In depth
It suits any situation where motion rendering is the fixed requirement and depth of field is negotiable — panning, sports, deliberate blur. Its weakness is that the aperture range is finite: demand 1/2000 s in dim light and the camera will open to its maximum and then simply underexpose, usually with a blinking warning.
Examples in practice
Panning cyclists at 1/30 s in shutter priority, the camera stops down to f/16 in sunlight to hold the exposure.