Definition: Printing processes such as offset lithography use printing plates to transfer an image to paper or other substrates. The plates may be made of metal, plastic, rubber, paper, and other materials. The image is put on the printing plates using photomechanical, photochemical, or laser engraving processes. The image may be positive or negative.
It's just one of the methods they use to make some of their cards. I think it's cool people can have a bit of something that was part of the creation process instead of just one of the thousands that the plates helped create.