According to the Fourteenth Doctor, the rules of play, or rules of the game, were the rules which "bound [the] entire existence" of the Toymaker and "governed" the Toymaker's domain, to the exclusion of the conventional logic of the rules of the universe.
As such, the Toymaker could ignore the ordinary "rules of the universe" and change reality according to his whim, in a process which wasn't even anything so scientific as "manipulating atoms with the power of thought", but he had to abide by universal, inviolable rules governing all game-play, including the principle of "best of three". (TV: The Giggle [+]Loading...["The Giggle (TV story)"])