Most Role-Playing Games operate under the principle that there are players and there is a Game Master. The GM prepares all the details of an adventure, and then "runs" the players through that adventure. This usually requires a great deal of preparation on the part of the GM and the handling of many details.
Mythic is different in that it requires no preparation from the GM. Mythic adventures are meant to be played off the cuff, with perhaps a few minutes of brainstorming to come up with the initial setup.
Mythic can also be played entirely without a GM. The same mechanics in Mythic that allow a GM to run an adventure without preparation also allows a group of players to do without the GM.