When you overclock your graphics card you set the settings in your computer to allow your graphics card to run much faster than it normally would.
Setting your graphics card to overclock is done in the BIOS settings and should only be done if you know what you're doing.
The purpose of overclocking the graphics card is to gain additional performance from the graphics card by increasing its operating speed.
Be aware that overclocking can harm your graphics card or anything else that you overclock so only do so if you really want too.