It's where you make a component, such as the CPU run faster than stock. For example, the X2 3600+'s stock speed is at 1.9GHz. You can overclock it to 2.8-3.0GHz. And it will perform like a more expensive processor, for much less. So you get more performance for your money. An X2 3600+ at 3.0GHz, can be near the performance of a stock e6600, which is $223. Isn't that pretty cool? A $70 processor can perform like a $200+ processor, through overclocking.