Overclock basically means set the clock (speed) to be higher than the stock (normal) speed. The purpose of doing this is to increase the performance of a pc. Overclock will produce more heat, to archive higher clock you need to deal with the heat first, to avoid toasting your processor obviously. In order for a processor to run faster, you will also need higher voltage. Overclock needs some basic setup which is hardware support, BIOS support, software support(optional, just for diagnose).