In the case of digital cameras, ISO sensitivity is a measure of the camera's ability to capture light. Digital cameras convert the light that falls on the image sensor into electrical signals for processing. ISO sensitivity is raised by amplifying the signal. Doubling ISO sensitivity doubles the electrical signal, halving the amount of light that needs to fall on the image sensor to achieve optimal exposure. In other words, if ISO sensitivity is raised from ISO 100 to ISO 200 while aperture is left unchanged, the same exposure can be achieved with a shutter speed twice as fast. The same is true if ISO sensitivity is raised from ISO 200 to ISO 400.
The slow shutter speeds needed for dark interior scenes leave photographs prone to camera blur. If you raise ISO sensitivity, you can choose faster shutter speeds and reduce camera blur. This is why people say that ISO sensitivity should be raised if lighting is poor.
ISO sensitivity can be set manually by the photographer or automatically by the camera.
These photographs of a cyclist were taken under low light. Low ISO sensitivities require slow shutter speeds for correct exposure, resulting in blur caused by subject motion. High ISO sensitivities allow correct exposure to be achieved at faster shutter speeds, making it possible to take photographs that “freeze” motion.
Raising ISO sensitivity allows faster shutter speeds, reducing blur caused by subject or camera movement. You may wonder why, if that's the case, you shouldn't simply always shoot at the highest ISO sensitivity setting, but in fact raising ISO sensitivity can introduce a type of image artifact known as “noise” into your photographs, making them seem grainy. Raising ISO sensitivity amplifies the electronic signal, which also amplifies any noise in the signal; as a result, the higher the ISO sensitivity, the more obvious the effects of noise on your photographs. The same is true of all digital cameras. We recommend that you raise ISO sensitivity only as high as needed to avoid blur.
Nikon (2019) DSLR Camera Basics. Retrieved from https://imaging.nikon.com/lineup/dslr/basics/01/01.htm