Ok, first stop is Google maybe the Wiki on clocking?
Basically though it is making a computer chip run faster than it is 'supposed to'
You see an Amd 975be for sale, they run at 3.6GHz Stock settings yes?
folks who overclock them will report speeds of 3.8GHz, 4GHz or higher, its the same chip, its just been made to run faster using some tricks,
it is fairly easy to get into but not something to rush into trying before you have learned a lot more, and the Overclocking sticky here, as well as others all over the net will help you understand a bit more,
sorry for what seems like a non-answer but its not a simple subject once you get into the details