From the Super Mario Wiki
How to Unlock
To access this level, the timer must end with the two digits '5' (that is, 'x55', where 'x' can be any number) when the player reaches the Goal Pole of any World 5 level. Once the level is completed, it can be accessed again by satisfying the above condition once more.
The level begins with Mario on land beside a Pipe Cannon. Entering it will shoot Mario to the rainbow course. Mario will land on a yellow moving platform. Upon landing, coins will jump out from below from both sides, similar to how a Cheep Cheep jumps. Ahead, more coins will begin to jump from the right. This is followed by a Roulette Coin Block. After that, more coins will leap out from the left, where a giant coin will then fall. A large Roulette Coin Block then appears. This is followed by more leaping coins, before Mario will have to jump down and end the level.