The shutter speed is responsible for exposure. It controls
the amount of time light is allowed to reach the film or sensor.
Shutter speed is measured in fractions of a second.
125 means 1/125th of a second, 60 means 1/60. Typical
shutter speeds range from 1 second to 1/1000.
A shutter speed setting for a bright, sunny day - using an aperture of F11 -
might be 1/125 second. A cloudy day might use 1/60 second with the same
aperture, exposing the film or sensor to light for a longer period of time.