ISO - The Cameras's Sensitivity to Light

ISO is  how sensitive the sensor in the camera is to light.

Low ISO = Low Sensitivity to Light
High ISO = High Sensitivity to Light

The ISO numbers double as they increase: 100 - 200 - 400 - 800, etc. Notice that this means each setting makes the camera twice as sensitive to light as the previous setting.

The ISO ranges from 100 to 3200 and above. There is also a "High 01" and High 02" setting.

Here are some links that go into greater detail about ISO.

https://photographylife.com/what-is-iso-in-photography

http://digital-photography-school.com/iso-settings

http://www.digitalcameraworld.com/2014/02/19/what-is-iso-camera-sensitivity-settings-and-the-best-ways-to-use-them/

You should always try to use the LOWEST ISO POSSIBLE to obtain the best quality image.