This may not be the best place to ask this question. However, I've only ever seen anime ones. I got an email from JList advertising Sword Art Online and I got a couple with a Galaxy Angel Vol. 1 Limited Edition box.
To me, pencil boards just look like plastic cards which, when flipped around (so everything is opposite), could be stuck to windows and allow light to shine though, but I doubt that's what they are for. What exactly are these pencil boards?