Answer:
The lower the film speed the more light (aperture) and time (shutter speed) you need to penetrate the film emulsion that contains the reactive chemicals that produce the negative to get a proper exposure, because lower speed film tends to have a thicker emulsion and more of those chemicals. The whole point of higher speed film is that it has a thinner emulsion, thus reducing the amount of light and time needed to produce the same image, but the typical result is "graining" because there are simply less reactive chemicals in the emulsion. With lower speed film the sooner you'll need a flash to compensate for the lack of light.
As an aside, the principle is similar in digital cameras, where the higher "ISO" results in digital graining, called "noise".