"The Raspberry Pi can now become a powerful, fast, and open digital currency bank."
Have you ever used a gift card to buy something from your favorite store? Do you find it more convenient to carry around a debit card instead of a checkbook?
All of these are good examples of a type of digital currency. You don't actually exchange any physical currency with the store, you simply have the store submit a request to the bank or account authority to subtract the final cost of the item from a known balance.
Digital Currency is simply substituting math for physical currency, trusting that the authority that maintains your balance (bank or local retail store) will honor the exchange of what is purchased.