From the Super Mario Wiki, the Mario encyclopedia
To access this level, the timer must end with the two digits '11' (that is, 'x11', where 'x' can be any number) when the player reaches the flagpole of any World 1 level. Once the level is completed, it can be accessed again by satisfying the above condition once more.
The level begins with Mario or Luigi entering into a green Warp Cannon and being shot into the sky. He then lands on a moving platform which continuously moves to the right. Upon landing, coins begin to fall from the sky. Two Roulette Coin Blocks appear, and more ten gold coins begin falling after that, along with a giant coin. Two more Roulette Coin Blocks appear, followed by a large Roulette Coin Block which changes between 10, 50 and 100 coins. Finally, more coins shower down, and Mario or Luigi must jump off of the platform. Falling at any time will bring Mario or Luigi back to the World Map, and the coins collected are saved in the coin counter.