Back in the olden days when using film, we called ISO, ASA (American Standard). You would choose different film with different ASA settings for specific situations. The lower the setting, the less light sensitive and the higher the ASA, the more light sensitive.
As Dave mentioned, the lower the ASA/ISO the more detail you'd get and needed a lot of light or exposure time. The higher just the opposite, less detail and less light required. Essentially, the low ASA/ISO film was for shooting polar bears at high noon in snow and the high ASA/ISO was for shooting black cats in coal cellars at midnight.
There were also other films available with mid-range ASA/ISO.
There were actually a few ways of getting a finer grain with high ASA/ISO film. How film worked was there would be a light sensitive emulsion on a piece of acetate. The thicker the emulsion the lower the ASA. The thinner the emulsion the higher the ASA. When developing film or printing film you had to be in a darkroom so all us old timers became very adept at doing things blind. We would have to in total darkness, take the film out of the camera, wind it on a spool that kept the film from touching, then put the wound film in a light proof developing tank, put the top on that and then you could turn on the lights.
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Next you'd mix up your chemicals and get it at a specific temperature. What the chemical essentially did was eat off portions of the emulsion that was exposed to light. The emulsion it ate off depended on how much exposure it had. There were charts for temperature and time needed to develop the film.
So one way to get finer grain with high ASA film was to develop it extremely slowly with the chemicals at a very low temperature so the eating away of the emulsion was very slow and not taking off large chunks quickly.
