The International Standards Organization has approved a standard measuring system for film/sensor sensitivity to light, which is named ISO.
ISO values are marked as arithmetical numbers on photographic films or in digital camera settings as follows:
50, 100, 200, 400, 800, 1600, 3200
The bigger the ISO number, the greater is the sensitivity to light.
Each time the ISO speed is doubled, the exposure must be halved, and opposite.
For example, if the correct exposure for a given scene will be 1/250s at F8 with a ISO 100 film/setting, the same scene photographed at ISO 200 would need a half of the initial exposure, meaning 1/500s at F16, to ensure a correct exposed image.
The example below is relative, as every camera / sensor has its own characteristics: