Depending on the camera and the complexity of the sensor you generally want to only use 400 and under ISO settings. Many cameras start to create noise at anything above that and many render nearly unusable images at 1600 or 3200 ISO.

What it is basically speaking is the speed of the film. This means the sensor is more sensitive to light at 400 than it is at 100 which means you can shoot in lower light settings with a faster shutter.

The rule of thumb though is to shoot with the lowest ISO setting you can get away with and still get a good usable image.