It means one with a wide maximum aperture. The lower the f-number, the wider the aperture because it's the ratio of iris diameter to focal length.
For example, these days f/1.4 would be considered very fast. f/2.8 is fast for a zoom lens and many prime (non-zoom) lenses of longer focal length. f/4 is pretty standard and anything from f/5.6 onwards is rated as being rather slow.
The aperture does two things - it lets more light in so you can take shots in lower light levels, and it affects the depth of field - i.e. how much is in focus. Wider apertures (smaller f-numbers) give smaller depth of field.
These days, with modern cameras having such good high-ISO performance, the light-gathering abilities of fast lenses are not quite as vital as they used to be - IMO of course.
