ISO is the sensitivity of the camera https://www.artlook.us/service/wedding-videography/ matrix. Designated in numbers, for example, ISO 100, ISO 200, etc. The higher the ISO, the less light the sensor needs to get a normal exposure. ISO is related to shutter speed and aperture.
Increasing the ISO by 2x raises the exposure by 1 stop. The higher the ISO, the slower the shutter speed is needed, and the more you can cover the aperture. ISO affects the quality of the photo: the higher the ISO, the more noise in the photo in the form of graininess; the lower the ISO, the better the photo.
ISO should be raised only when there is not enough shutter speed to take handheld photographs. On full-frame sensors, the graininess with raising the ISO is less pronounced, so on full-frame CZKs you can shoot at a higher ISO without significant loss of quality than on cropped CZKs.
On modern cropped CZKs, you can raise ISO up to 800 without a significant loss in photo quality. ISO 1600 will still be tolerable, but a little graininess is already noticeable. At ISO 3200 and above, there will be a significant loss in quality.