ISO is how sensitive the  camera is to light. so for example an ISO set at 100 would need more light to have the picture turn out better. If you're ina darker room then you would want the ISO to be on a higher setting for example 1600 would be best in a dark area but the picture would turn out to have a grainy texture.

In these photos all I did was change the

setting of the ISO which woud cause the photo to be darker or lighter depending on how I wanted it to turn out.