Iso Is is essentially the feature that allows you to either brighten or darken a photo , a low iso such as 100 will give you a darker picture whereas an iso such as 12000 will give you a much brighter image however having a higher Iso also has a drawback being that your image will end up being more grainy.Iso is just measured in numbers a low iso means a darker image and a high iso means a brigher image.You typically want to use a high iso when your in a dark environment and you want to use a low iso when your in a wel lit environment