FYI You posted this in the ancient threads area. This area is from long ago, look at the date of the original post. Anyhow, this area was from a time long ago.
To answer your question, the yellowbox is a mod that allows you to correctly calibrate your speedo. The bike's speedos are off (read faster) from the factory by a certain percentage. So say you are going 75 (indicated) you are likely only going 68 or so.
Do a search in the other areas for yellowbox...you should find everything you need out there.