Overclocking is the manual changing of a CPU's clock speed. It means to make your cpu faster then the original specs. This often comes at the price of heat. The faster you try to overclock your cpu, the hotter it will run. And if you don't know what your doing you can easly fry your cpu.
As far as how to do it, I suggest you go to an overclocking website. There are numerious ways to do it depending on your specific cpu/mobo and it would be impossible to give a "general" how-to.