It means that the head tracker can follow you as you turn all the way around. The fixed-head-position field of view is rather narrow compared to real vision -- equivalent to a 105" screen at 12', so less than 45 degrees, presumably measured diagonally accross the screen. Still, I don't think think that's a worse angle of view than looking at even a pretty large monitor. The innovation here is that head tracking provides a simple and intuitive way to look to either side. I wonder how well it will work in pratice -- I've heard that the image has to change in very close synchronization to head movement to feel comfortable. Well, one probably learns how fast one can move one's head.
