Film speed ratings are used to indicate the relative amount of light necessary to give a proper exposure.
A normal film will be rated at ISO 100. A film rated at ISO 200 will give a proper exposure with only half the amount of light compared to the ISO 100 film, enabling you to shoot in lower light or with a smaller aperture or faster shutter speed.
The ISO 200 film would be referred to as a ‘faster’ film.
Back to my original question. The faster the film, the more prominent the grain structure will appear in the photo. However, if the film is too fast, the individual grains clump together to form spots that are visible to the naked eye, especially when you blow the photo up to A4 or larger from a 35mm negative.
By the way, “ISO” is an acronym for International Organization for Standardization.