The number on the side of the box your film comes in, which usually varies from 100 to 400 on most common films, is what is called the ISO Rating of the film. This rating is an index of the speed of the film, with a larger number indicating a faster film. Well, to understand this, one needs to have an idea of how film works.
Silver bromide is quite sensitive to light, and undergoes a chemical reaction when light hits it.
Then when the film is developed, the silver bromide is again chemically changed, making the areas struck by light become dark, and the negative image is created. Now the speed is based on the size of the silver bromide used to cover the film. Thus, the image is produced very quickly, and the film is called a fast film. However, if the silver bromide is in very fine particles, more need to be exposed before the image is produced, and the film is much slower. What this means, then, is a fast film, with large chunks of the silver bromide, has a lower quality than the slower film.