The Piggy Bank is a limited feature and may only be available on a limited basis.
When the feature is available, the Piggy Bank will fill with gems when you win a level, and it adds more when you win with moves remaining. You can purchase the piggy bank with real money to receive the gems, which will add the gems to your total balance at the top of the screen.
If you are not interested in the Piggy Bank and prefer to opt-out, please ignore the feature.