When you overclock your computer it means that you set your Memory and CPU to run at much higher speeds than their normal speed grade.
Lets say your CPU with your computer runs at a speed grade of 2.80 GHz so if you overclock that CPU you would be running that CPU at any speed higher than the 2.80 GHz.
People overclock their computers CPU to speed up their computers for serious tasks but overclocking can damage your computer.