ISO determines the sensitivity of the camera’s sensor to the light entering the camera through the lens. The higher the number, e.g., 100, 200, etc, the more sensitive the sensor will be to the light entering the camera. The advantage of higher ISOs is that you can use slower (lower) shutter speeds and smaller apertures (higher numbers) given the same amount of available light. The disadvantage of higher settings is the potential for “noise”, which is that grainy effect in which you start to see the individual pixels.
With lower ISO settings, you will need one or more of, greater available light, slower shutter speeds, and wider apertures, but you will get less noise. The bottom line – don’t boost the ISO unless you really need to.