When you take a picture, it opens briefly and allows you to control the amount of time the light passing through the lens remains in contact with the digital image sensor. It is activated by the camera's release button.
The shutter speed refers to how long it remains open and the usual range of speeds is: 1, 1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000, 1/2000, 1/4000 sec .
Except for 1, these time measurements are fractions of a second where each increment either halves or doubles the time of the previous one. If we take 1/60 of a second as an example, then it is half as much exposure time as 1/30 of a second, but about twice as much as 1/125 of a second.
The doubling or halving of the
the speed settings compliments the aperture f-stop scale in terms
of the exposure
given to the image. The relationship between aperture
and shutter speed is know as reciprocity. What it
means practically is that
the following combination of settings will give the same exposure of
1/30 of a second at f/11 is the same exposure as
1/60 of a second at f/8 or
1/125 of a second at f/5.6
and this allows you to give correct exposure over a very wide range of brightness conditions as well as to control the depth of field of the image.
In addition to giving correct exposure, the speed setting of the shutter can be used to record motion either as frozen or blurred to give sense of action.