It stands for "Really Free" Online.

Codemasters announced today that their Science Fiction MMOG RF Online will be moving to a free to play payment model.

Codemasters Online is pleased to announce that RF Online is now a completely free-to-play MMORPG with the full game now available for download with no monthly subscription required.

RF Online is an epic mix of traditional fantasy and futuristic Sci-Fi set in the deep space galaxy of Novus where three races compete in an ultimate struggle for power and resource. Celebrating the games re-launch, new players can now also experience the all-new free expansion, Episode II: Pioneers of Novus available for free to all players of the game.

Click here for the original announcement.