At some point after you've purchased that entry level sub $1000 dSLR kit with a couple of consumer grade zoom lenses, you're going to want a faster, sharper lens. By faster I mean one with a larger f-stop than the typical f3.5 you're getting at the wide angle setting of the lens in the kits. Remember, a faster lens lets in more light, so you can you can use a faster shutter speed. That's where the term comes from.