From the Super Mario Wiki
World 6-Rainbow is the sixth of the rainbow courses that appear in New Super Mario Bros. 2. To access this level, the timer must end with the two digits '66' (that is, 'x66', where 'x' can be any number) when the player reaches the flagpole of any World 6 level (excluding Fortesses or Castles). Once the level is completed, it can be accessed again by satisfying the above condition once more.
The level begins in a volcanic area on the ground. When Mario heads to the corner of the stage, coins begin to rain down from above. Entering the Cannon Pipe in the center of the stage takes him to the rainbow level itself.
The upper level begins with a normal platform. This is followed by several bouncy mushroom platforms and coins that rotate in circular formations. Near the first platform after beginning, a Giant Coin descends. Afterward is a Red Ring, which spawns the eight Red Coins amongst the rotating coins following. The level ends with more platforms and several rotating coin formations above the pit that must be used to exit the level.