CHG_CANON wrote:
A "fast" lens is photographer slang for a wide maximum aperture, preferably a fixed maximum aperture, say f/2.8 or wider, where f/2 would be an even better minimum 'wide' value to warrant this "fast" tag. Think f/1, f/1.2, f/1.4 for this "fast" slang application, although f/2 for 200mm is a pretty sweet lens.
You can find longer discussions, with examples, from a simple google: what is a fast lens
With digital cameras, increasing the ISO can compensate for a "slower" lens in low light. Faster lenses allow you to shoot with shallow depth of field, sometimes desirable.