I'm not sure which camera you use, so it may be different than my Nikon D70s.  First you have to understand that ISO was first used with film, example film cameras. It expresses how sensitive a particular film was to light. The lower the ISO number, the less sensitive the film to light. For example, you might use a 200 ISO film in the daylight, or use 800 ISO film at night. Now we jump to the digital world.

When we talk about ISO with digital cameras, it gets a little trickier. Keep in mind, in digital cameras, we have a sensor that replaces the film. So one might think if I set the ISO on my camera, I must be setting the sensitivity of the sensor? Maybe yes, maybe no. With my Nikon, the sensor has an equivalent sensitivity of around ISO 200... this cannot be changed. If I set the ISO to anything else, it doesn't actually change the sensor at all, but rather tells the camera to amplify, (increase the exposure) after the picture is taken. This conversion (I call it in-camera postwork), only happens if I'm shooting in Jpeg picture format. I shoot soley in Raw, so setting the ISO has no effect on the pictures.

Now some cameras will actually change the sensor's sensitivity. Perhaps yours does?