The short story:
TV's typically display pictures as interlaced images with a 480 resolution. This is how the cable signal comes in. So every refresh, refreshes half of the screen. Thus it takes two refreshes to redraw the whole screen.
Progressive scan is not interlaced. It redraws the whole screen in one refresh. You'll commonly seen PS as 480p or something similar.
To use PS, you need a source that can transmit in that format. DVD players, HDTV stations, and satelite TV decoders are the only two that I am aware of as for now. To display it, you need a HDTV.
Hope that helps.