The ISO setting is the sensitivity of the sensor to light. The higher the number, the more sensitive it is to light. When using film for photography, the entire roll was the same ISO (such as ISO 200 or 400). There is a temptation to use a high ISO setting in a low light environment. However, if you use a high ISO setting, noise (or interference) gets introduced. A better solution is to increase the brightness by editing the photo. The best setting is the lowest setting that works. Generally, an ISO of 200 or 400 should work for many situations.
This also works with the aperture and shutter speed settings.