Ok so when you say "fast glass", what does that mean exactly?
Is that a type of lens?
"Fast glass" refers to a lens with a large aperture that allows lots of light through.
Aperture is measured in f-stops. The smaller the number the "faster" the lens.
An f-stop is the focal length divided by the diameter of the lens.
A fast lens is typically f/2.8 or faster although there is no official definition and it depends on the focal length.
Telephoto lenses with an f-stop of 5.6 or less are also sometimes called fast lenses because they're fast for the focal length that they are capable of.
A Canon EF 500mm f/4L IS USM lens for example can also be referred to as a fast telephoto lens.
f/1.4 is very fast and usually very expensive and normally only found in lenses with a focal length of 200mm or less.
The lens in itself isn't fast but the larger aperture allows for faster shutter speeds hence the term "fast glass" or "fast lens".
Because the lens lets more light through, a slower ISO can be used with a faster shutter speed.
That in turn helps avoid motion blur due to a slow shutter speed and sensor noise problems that are experienced when using high ISO settings (film speed).
This page explains F-Stops quite well:
http://www.uscoles.com/fstop.htm
Wikipedia have a page on lens speed:
http://en.wikipedia.org/wiki/Lens_speed