Every camera has a different range of ISO values (sometimes called ISO speeds) that you can use. A common set is as follows:
ISO 100 (low ISO used in sunny or bright conditions)
ISO 6400 (high ISO but most modern cameras can go from ISO 50 (L) and up to 102400 (H).
Quite simply, when you double your ISO speed, you are doubling the brightness of the photo. So, a photo at ISO 400 will be twice brighter than ISO 200, which will be twice brighter than ISO 100.