It's basically just drawing a rough comic book-like version of the video you're making. You'll have to get the placement of basic objects (tables, chairs, whatever) as well as the camera angles (over the shoulder, wide shot, etc etc). It helps me to make little notes as well as write what the person would be saying, what music is in the background, whatever. If you're not great at it, try storyboarding a scene from your favorite film. That's how I learned. :]
