It means everything that will end up in the picture can be seen in the viewfinder. Less than 100%, there can be something - trash on the ground, etc. - outside the VF coverage, that you don't see until you review the image. Basically, it means you can more accurately compose your picture.
Its pretty hard to explain all I can say is go to a camera shop that sells 1D series cameras and look throught the view finder and you will understand. its a massive difference. even over the 5D2 the 1D VF is soooo much better.
