ISO is basically how sensitive to light the film is. A low ISO number mean low light sensitivity which requires more light to expose and a high ISO number has high light sensitivity, meaning it needs less light for exposures.
The ISO refers to the level of sensitivity that a film has, and typically can be found in variations of 100 (Kodak Ektar), 160 (Kodak Portra) 200 (more widely adopted but typical of Kodak Colour Plus or Kodak Gold) and 400 (Kodak Portra, Ilford HP5 and many more) and then more bespoke 800 (Portra again!) and 1600 (Fuji Superia Natura) films.
The lower the number, the less sensitive the film is to light. 100 then would be ideal for sunny days and means that you are able to use a more wide open aperture or slow shutter speed in bright conditions. On the down side this will also mean that you will find less usable light as the day progresses into dusk or evening.
200 and 400 film are more standard for diverse conditions but will require more attention in brighter conditions as not to blow out the exposure, typically meaning that brighter days you will be shooting around f.8/f.11 and shutter speeds between 500 and 2000. The upside is that it will allow you to shoot for longer in natural light and has more tolerance for fluctuations in weather, holding itself well in overcast conditions. 800 to 1600 will often be a go to film when shooting indoors with low levels of light where flash cannot be used but is often on the higher end budget wise.
100 then can be thought of as more specific to portraits (although not always). When taking a portrait and shooting wide open for bokeh effect and subject separation from background (using a low aperture/f.stop number, often 1.8 or 2.8), you may find with a 400 film you are over exposing unless you have a high shutter speed to compensate. Likewise, 400 is more rounded for landscapes, in order to get a whole scene in focus, around f16 to f22, if 100 ISO film was used, the shutter may drop down to 60 to 30 or below where shake can be an issue. Horses for courses.
On your camera you will have the ISO selection dial (often marked additionally as ASA). By setting this ISO/ASA to match the film that you have loaded, you are setting the camera up to factor in this sensitivity and how it will affect the aperture and shutter speed. In the hierarchy of ISO, Aperture and Shutter Speed, ISO is king and must be factored in first. Whilst negative film can often be forgiving to incorrect exposure, even if your aperture and shutter speed is balanced, if the ISO is incorrect, for example if it is not changed when a different film is used, the exposure will not be correct. Some modern cameras will read ‘DX’ films and automatically select the correct ISO automatically.
Creatively, the less sensitive a film is, again 100 often being the lowest so least sensitive, means a finer grain and smoother look to the texture of the film. This in my experience is particularly noticeable with black and white film.