What is overclocking? (in layman's terms)?

I am really curious as to how overclocking works, but I am not as technical as many people are with computers. My HD videos always play with lag... the audio is fine but the video stops. I have every GOOD codec imaginable and I have a high end graphic card put in my system that is up to date but I still get horrible results. Not to mention I have 8mb of RAM... how can overclocking help this???