You can change the camera’s ISO (sensitivity) setting manually via this button or set it up to choose the ISO automatically. To choose a value, hold down the button and turn the rear command dial. To let the camera choose the setting, hold down the button and turn the front dial to choose between manual and auto ISO control.
The auto ISO mode is actually very useful. You can set this up to use a maximum ISO value and a minimum shutter speed – in this way, you make sure the camera only increases the ISO by enough to avoid camera shake, and only does it when necessary. This means it uses the lowest ISO it can for each shot, which is much better than choosing a blanket ISO setting which may be higher than you need.