ISO measures the sensitivity of the light sensor,  which is responsible for gathering light and transforming it into an image. The lower the number the lower the sensitivity of the film and the finer the grain in the shots you’re taking. In darker situations a higher ISO setting is used to get fast shutter speeds.

ISO Speed Example:
ISO 100 – 1 second
ISO 200 – 1/2 of a second
ISO 400 – 1/4 of a second
ISO 800 – 1/8 of a second
ISO 1600 – 1/15 of a second
ISO 3200 – 1/30 of a second

The ISO sequence is: 100, 200, 400, 800, 1600, 3200, 6400 and etc. Each step between the numbers effectively doubles the sensitivity of the sensor.