ISO is the unit for the light sensitivity of the camera’s storage medium (for analogue cameras this is the film speed, for digital cameras it is the sensor speed).
The higher the ISO value, the more light-sensitive is the sensor or film, i.e. the less light is required. This makes taking pictures in the evening and even during the night possible.
Disadvantages of a high ISO setting: Reduction of image quality in the form of blurring and noise.