This was the first of these things I've seen, and the only one I have. What exactly is it?
As near as I can tell by squinting over the microscopic type in Beckett Baseball, the first of these may have been issued as inserts in 2004 Classic Clipping (a Fleer product). Those were called "print proofs" so may not have been the same thing but they did come in the four printing colors - Black, Cyan, Magenta, and Yellow. There were four for each card in the set, each in one of the four colors.
In 2005, most Bowman brands, Finest and the base Topps brand had "print plates", again 1 in each color for each card. The base Upper Deck set also had them.
In 2006, virtually every Topps produced set had them, several Fleer sets had them and a couple of Upper Deck sets had them as well. They were also available in various sets in 2007 and 2008.
Since this is called a "print plate" I have assumed that this thing was used to print the actual card. I'm not an expert on printing processes or colors, but apparently the four colors mentioned above can be combined to make a full color image. The colors are usually abbreviated CMYK (K for black). So what I have is the magenta plate.
Baseball cards are printed on big sheets. So this thing must have been cut from the magenta plate. It's hard to tell in the scan but the thing has rounded corners. It's also made of aluminum.
Another thing: The most common form of printing is called offset printing. In this process, the printing plate is inked from a rubber roller. Parts of the plate (the part which the printer wants to ink) is attractive to the ink (that is the ink sticks to it), while the other parts are not. The ink on the plate is then transferred to a rubber pad. The image on the pad is then transferred to the print media (the paper, or in this case, a baseball card sheet). This explains why the image on the printing plate is in the same orientation as the card (and not a mirror image).
So how do the different colors get made? I probably knew something about color theory at one time but I looked this up (like the other links) on Wikipedia. See that the bottom of the card is blue? The magenta plate is inked there as well. The cyan plate would also be inked there because cyan and magenta together give blue. The yellow and black plates would have no ink across the bottom.
If the printing plates are cut up and distributed, does that mean that no more cards of that set can be printed? Probably not. I think new printing plates can be made, especially in this day and age when the original images are stored digitally on a computer.