The shutter speed is the amount of time that the sensor is exposed to the incoming light from the scene you are photographing. The number we assign to this parameter is expressed as a fraction of a second, for example
The last one, “15”, means that the sensor is exposed to light for fifteen seconds.
In most cameras, the time the sensor is exposed to light is controlled by a mechanical shutter (hence the term, “shutter speed”). Think of the shutter as a curtain that is normally drawn shut across the sensor. You expose the sensor by opening the curtain and then closing it back again. Of course, in cameras, this action can take a tiny fraction of a second!
The longer you keep the shutter open, the more light you allow to hit the sensor.
If one only is allowed to change the shutter speed, then one would need to have a “fast” shutter speed on a bright and sunny day. Conversely (again, all other parameters untouched), if one is in a dark bar at night, one would need to use a “slow” shutter speed to expose the picture properly.
To double the amount of light that hits the sensor, you slow down the shutter speed by a factor of 2. These shutter speeds, in order, are each twice as fast (one stop) as the one preceding it:
1 sec, ½. ¼. 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000, 1/2000
(Note that there is a bit of fudging here. 1/15 is not exactly twice as fast as 1/8, but it is very close. 1/15 and 1/125 have been chosen, historically, to make the shutter speeds round numbers.)