This question already has an answer here:
With conventional cameras your camera had a maximum shutter speed ( typically 1600 ), and the film you used had an ISO speed rating at which it was meant to be able to get a proper exposure under "normal" lighting conditions. Essentially this was a way of rating the sensitivity of the film. While you can push a given film to a higher than rated speed, unless you have lots of light, the quality of the picture is going to suffer due to being underexposed.
Why do digital cameras not seem to be given an ISO rating? It looks like the manufacturers are trying to misrepresent the maximum shutter speed as an ISO rating and make ludicrous claims with very high numbers for the sake of specsmanship, but trying to use those high speeds invariably results in terrible, underexposed pictures ( due to the very small dynamic range that has to be highly amplified to get anything at all ).
One manufacturer could have a terrible sensor but push the maximum shutter speed to 100,000, and you're not really going to be able to use anything more than 200 in lower light conditions, while another could only have a maximum shutter speed of 3200, but you can actually use it at 1600 in low light and not get underexposed pictures. How can you tell the difference?