ISO in digital photography measures the sensitivity of image sensor. It’s also one of the basic ingredients, together with aperture and shutter speed in order for you to get that desired shot.
Set at the same Aperture number and shutter speed, this is the result of me experimenting with ISO.
As you can see, the higher the ISO, the brighter the image. So, for indoors, I recommend using ISO 400, for outdoors ISO 100.
I’m still a noob at photography, but I hope I helped you understand ISO.