What does speed have to do with it?
The speed of a lens and how ‘fast’ it is refers to the maximum aperture of the lens. The larger the maximum aperture, the more light entering the camera, the faster the shutter speed can be set, the faster the lens.
Fast lenses can be advantageous over slower ones in certain shooting conditions. They excel at night and in low light as they allow more light in to expose the film, increasing the chance of a faster shutter speed and avoiding the use of a tripod.