Understanding ISO
ISO controls how strongly the camera amplifies the signal from the sensor. Raising it can make a usable exposure possible in lower light, but it usually brings more visible noise and less image quality.
What ISO does
On a digital camera ISO changes how strongly the captured signal is amplified or processed for the chosen exposure. Raising ISO can let you use a faster shutter speed or smaller aperture in lower light, but usually brings more visible noise and can reduce usable dynamic range.
Do not be frightened of high ISO
A sharp photograph with some noise is often more useful than a clean but badly blurred one. The useful ISO range depends on the camera, the light, the final display size and how much editing you intend to do.
Auto ISO
Auto ISO can be extremely useful. The camera chooses ISO as conditions change while you control other settings. Later in the course you can decide when to limit its maximum value or minimum shutter speed.