ISO is simply how sensitive the sensor is to the light that it records. 1 stop increments of ISO are as followed: 100,200,400,800,1600. In the film age, you would have to physically switch rolls of film out that have different ISO speeds, but now in the digital age, you can simply select it on your camera.
Shortly I will post something that relates every in basic knowledge together (aperture, shutter speed, and ISO)
