Most TV programs, VCR tapes, laserdiscs etc that you watch are interlaced. What this means is that lines 1 3 5 7 9, etc etc are drawn, then it goes back and draws 2 4 6 8 10 etc until each frame is complete. It does it obviously so fast that you can't tell, but what a progressive scan DVD player does is draws each line one after another, so it goes in order, 1 2 3 4 5 6 etc until it's finished. I've heard it is a noticeable improvement on cartoons/movies which have animation such as Who Framed Roger Rabbit as the cartoons movement is much more fluid. The actual improvement will depend on how sensitive to these things that you are, but not only do you have to require a player which supports progressive scan (the PS2 is far from being able to support it and never will be able to, simple cables do not change the player to progressive) and also your TV has to have progressive INPUTS to display a progressive signal. So a couple things are required before you can actually watch DVDs this way.
dm