The ISO value refers to the sensitivity to light of the digital sensor inside your EOS.
The sensor is composed of millions of tiny photocells. These generate tiny electrical charges when exposed to light. To increase the sensitivity of the sensor these charges are amplified. Unfortunately, in addition to amplifying the main charges, some background charges (‘noise’), are also amplified. At higher ISO values this noise becomes visible in the images as a type of grain effect (right hand image below).
As ISO value rises, so does noise
Low ISO values give little or no noise but as you increase the ISO you will have to balance the benefits of high ISO values with your own preference for the amount of noise you are willing to accept in your final result. You can set ISO using the button in front of the LCD panel on the top right of your EOS.
Increasing the ISO value is not just something to do when light levels are low and you have left your tripod at home. Using a fast shutter speed often means setting a wide aperture to compensate, but introduce high ISO values into the equation and it is possible to use a fast shutter speed with a small aperture.