circles a module is an adventure that is designed by someone other than the person game master
ing it. The term is usually applied to published
adventures, however it can be used to describe any adventure that is written down and circulated beyond the desk of the person who designed it.
As far as I can tell this usage originated as a term for the official adventures published by TSR for the original D&D game, but for all I know it could have been used earlier than that to describe scenarios for miniature wargames.