You goal is to collect caterpillars and help them transform into beautiful butterflies. To accomplish this, you must match the correct kinds of tiles to get caterpillars, and then continue making the right matches to feed your caterpillars and help them grow.
To help you achieve this goal, Mahjong Butterfly offers you the ability to take and hold a tile in reserve, allowing you access to a previously inaccessible tile without having to remove a valuable piece you want to play later. As you collect butterflies through the 47 different games, you gain more entries in your collection of butterflies; a nice reference that shows you more about the butterflies you've raised.
If you are ready for a more engaging Mahjong experience, then take some time to play with butterflies!