The ISO is system is used to measure the sensitivity of digital imaging systems. It basically controls your camera's sensitivity to light.
Simply put, the higher the ISO number on your camera the more sensitive your sensor or film is. In other words if you're trying to shoot something in low light situations you will need to shoot at a higher ISO number. If you are shooting in very bright conditions you will want to shoot with a lower ISO number.
Generally speaking the higher the ISO you more noise you'll get in your image.
Here is a great video about ISO:
This video discusses ISO (starts at 4mins)
This video explains ISO in detail:
This video explains ISO in a fun way:
Here is another video on ISO:
Here is another video about ISO:
Video by Tony Northrup
Video about Film Grain Vs Noise
Video by Matt Granger