Basically testing just how fast xyz really is in comparison to others.
If you were to test your PC for gaming purposes you would use something like 3dMark 2001SE (
www.3dguru.com, or
www.madonion.com I believe) and it will give you a score... depending on that score you could compare it to other machines to see how fast your machine is or how well things are tweaked.
For instance, if you took your machine with a Fresh install of win2k nothing added and got a score of 5000.. you then updated all of the drivers and did some tweaking and overclocking you could possibly raise your score up to 5500 or so.. just a way to track how well something is running in comparison to something else.