When you use your Magenta Pay Card, transactions may not be whole euro amounts (e.g., €5.40). The difference to the next euro (e.g., €0.60) is called "spare change".
If you enable automated deposits, this spare change is saved in your Piggies.
If your wallet lacks funds for such savings during a transaction, the amount will not be saved, but another attempt will be made the next time you use your Magenta Pay Card.