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ISO (or what use to be called ASA) is the "speed" of the detector. With film the lower the ISO, the slower the film but the higher the resolution with less grain(noise). The trade of for faster and hence more light sensitive film is less resolution and more noise in the final image.
The same thing applies with digital cameras. With your camera in bright sun it probably doesn't have the ability to do a fast enough exposure when you have the ISO set at 400. The end result is an over exposed image. You are better off setting the ISO at a lower number. This will result is less overexposure, less noise and a better picture.
Film/CCD need a certain amount of light to hit them to achieve what we would perceive as a well exposed picture. This is determined by the shutter speed, aperture of the lens and the speed or ISO of the film/CCD. Doubling the ISO has the same effect as doubling the exposure time or opening the aperture up 1 stop.
With astro photography things get a bit more complicated. We usually work with under exposed images due to the dim objects being photographed. This takes long exposures-usually many minutes to hours. ISO change for a CCD is essentially performed just by amplifying the signal that has been received and not by actually increasing the amount of light detected. This will increase noise as well as the signal. Often it is better to use a lower ISO setting that allows you to have less noise in your final image and use other techniques to improve the exposure.
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