Shutter speed is one of the three aspects of the exposure trinity and the second part of the exposure value composite. Shutter speed denotes the length of time for which the shutter curtains remain open to accept light.
It is curiously mentioned as shutter speed but the fact is that shutter can remain open for several minutes at a time depending on the requirement of the photographer (long exposure photos).
Shutter speed is expressed in seconds as well as fractions of a second: 1/500, 1/250, 1/125, 1/60, 1/30, 1/15, 1/8, ¼, ½, 1”, 2”. This is obviously not the full chart. The chart extends in both directions.
Varying the shutter speed, i.e., using a slow shutter speed or a fast shutter speed will depend on the creative requirements of the photographer. Increasing the shutter speed will freeze movement while decreasing it (dragging it) will capture motion blur.