They are cloth bags, usually made with cotton materials and thread, that you put potatoes in and nuke them. I guess they come out moister this way. They also seem to be a big hit at some craft fairs, too :D
I don't know if I would want to cook more than 2 at a time in these... but that is just me :wink:
They talk here about different foods used in these bags.
http://www.comfortbags.com/micobagforpo.html
http://www.inthecountrycollectibles....toeBakers.html