Shutter speed means how long the shutter is opened or the duration of light.
Shutter speed mainly depends on the nature of light or the Nature of the subject.
When our subject is moving and we want to freeze our subject then we use a fast shutter speed,
and when we have a good nature of light then we use a fast shutter to maintain proper exposure.
on the other hand, when our subject is in low light then we use a slow shutter speed.
The measurement of Shutter speed is in Second or Fraction of second.