Lens speed refers to the maximum aperture diameter, or minimum f-number, of a photographic lens. A lens with a larger maximum aperture (that is, a smaller minimum f-number) is called a "fast lens" because it can achieve the same exposure with a faster shutter speed. Conversely, a smaller maximum aperture (larger minimum f-number) is "slow" because it delivers less light intensity and requires a slower (longer) shutter speed.
A fast lens speed is desirable in taking pictures in dim light, or with long telephoto lenses and for controlling depth of field and bokeh, especially in portrait photography, and for sports photography and photojournalism.
Lenses may also be referred to as being "faster" or "slower" than one another; so an f/3.5 lens can be described as faster than an f/5.6.