Il be honest aswell, Jim in that I'm no expert either but in laymens terms I always understood the ISO rating of the old film camera's we all used to use, as their sensitivity to the amount of light needed for a given exposure time.
When I used to do abit of night time photography, a higher ISO of say '400' meant I could use a faster shutter speed and would then avoid having to use the tripod so much or longer exposure times to get the same (hopefully decent) result. The trade off though, is that the higher ISO figure you use, the more 'noise' you get in the picture, or 'graininess'. (if thats a word!)