In photography ISO is the sensitivity the sensor has towards light, it is the third and final aspect to a cameras light settings (Aperture, shutter speed and ISO). ISO controls your photos brightness.
ISO is artificial light which means you can’t just add it in without trade offs, too much ISO can lead to noisy shots, ISO should be the last setting adjusted to get brightness.
Shooting a sunrise or sunset in low ISO will produce better colors in most cases as it increases the dynamic range.
Not surprising ISO is measured differently to the other two adjusters… ISO 100 is low which would make a shot very dark through to a high ISO 6,400 which would make your image very bright.
A good rule of thumb is just to stick with your cameras base ISO level and adjust shutter speed and aperture to lighten a shot. With actions shots that need a fast shutter you will have to adjust ISO.
Adjusting ISO is usually allowed in a manual shoot mode on your camera, most of the time ISO will be the default to auto adjusting.