Auto ISO is a setting now provided in most cameras and it can prove to be very useful.
When we used film we were commited to the ISO setting of the film until the film was finished and then we would place a higher ISO film in the camera and commence shooting again.
With the introduction of digital imaging we can now change the ISO at any time we desire. Most cameras now also have an auto ISO setting.
In general people tend to avoid the Auto ISO feature on the latest cameras in fear that the camera will automatically increase the ISO up and introduce noise into the image. It is a known fact that higher ISO settings do introduce more noise. While this is very much a concern, the Auto ISO feature is very useful and generally applies conservative settings.
Also the Auto ISO setting has various options to limit the range of settings to ensure the quality of images is preserved.
With Auto ISO the camera attempts to use the lowest level of ISO setting and the camera will automatically adjust the ISO, as the shutter speed is reduced the ISO will increase, also the lens focal length will have a bearing on the change in ISO.
Essentially as the amount of light diminishes the camera ISO will increase. This is a very powerful tool to use in action shooting, where the camera will automatically compensate as the available amount of available light reduces.