What is ISO?
In film photography ISO was the indication of how sensitive a film was to light. In digital photography, ISO measures the sensitivity of the camera image sensor.
ISO is measured in numbers – ie. 100, 200, 400, 800, 1000 and up. Some modern digital cameras ISO range can go over 50,000!
The lower the number the lower the sensitivity of the image sensor and a finer grain of your image and the higher the ISO number the more sensitive your camera sensor becomes and the more grain will appear in your image.
With higher ISO numbers your camera sensor becomes more sensitive to light which allows you to use your camera in low light situations like astrophotography.
Through animation in this video Apalapse explains ISO.