A history of franking

Franking (or “franks”) are any and all devices or markings such as postage stamps (including those pre-printed on postal stationery), printed or stamped impressions, codings, labels, manuscript writings (including “privilege” signatures), and/or any other authorized form of markings affixed or applied to mails to qualify them to be postally serviced

A nice little history of franking here from Wikipedia.