That's basically it. ISO 100 is an approximation to ISO 100 film in sensitivity (the same aperture and shutter speed should give the same exposure, in theory). Of course it's slightly more complicated than that due to sensor noise, marketing "statistics", and the like, but some sites like DPReview will even go out of their way to test the accuracy of the ISO ratings of each camera. I suppose the idea is that if ISO 51,200 film existed, it would have the same sensitivity to light as a digital sensor set to ISO 51,200.