higher ISO = more sensitive to light = more noise / grain
lower ISO = less sensitive = less noise / grain, if any
it's another variable for you to use to get the result you want. If you want to learn other bits first, keep it on 100 if it's sunny out, 400 if cloudy / dusk, higher when needed / indoors
to set the sensitivity of the sensor
higher ISO = more sensitive to light = more noise / grain
lower ISO = less sensitive = less noise / grain, if any
it's another variable for you to use to get the result you want. If you want to learn other bits first, keep it on 100 if it's sunny out, 400 if cloudy / dusk, higher when needed / indoors