ISO

ISO

ISO is the third exposure control in the triangle. It is used to increase or decrease the cameras sensitivity to light. If you are in a dark room or place and your camera is set to the settings that you want, such as if you were doing a concert and the aperture was at the lenses minimum and your shutter at 1/60th, if your exposure meter is still reading -2ev then you need to compensate for this by boosting the ISO! the boosted ISO will then even your exposure. 
If your exposure was under by 2 then increase your ISO by 2 increments to compensate.

ISO 3200

ISO 400

Raising the ISO is a great advantage because in the film days, you had a set standard of film such as 100, 200, 400, 800 and this caused a pain as you would not be able to change it. The huge disadvantage of raising your ISO is the quality degrades as you increase the value. A higher current passes through the sensor, this creates "noise" and the image will lose some colour and detail.

ISO is an extremely useful tool in the manual controls, it is useful when you have the settings you want such as shutter and aperture, then if the image is too dark, you can boost your ISO, but be careful and remember the effects.