The board is nine small tic-tac-toe boards. Win three small boards in a row to win the game.
Where you play inside a small board decides where your opponent must play next. Play in the top-right cell, and your opponent has to play in the top-right board.
If you're sent to a board that's already won or full, you can play in any open board.
The boards you're allowed to play in are outlined in yellow.