Interest rates are currently at a historic low. As a result, the European Central Bank and the Deutsche Bundesbank are not currently paying interest on funds deposited with them, but are charging interest.
The portion of customer deposits in current accounts that is not invested elsewhere by banks is parked overnight at the Bundesbank. The Bundesbank currently charges an interest rate of minus 0.5% per year on these deposits. The purpose of this negative interest rate is to encourage customers and banks to not leave their money in current accounts, but to invest it instead.
By charging a deposit fee (Verwahrentgelt), banks pass on the costs to their customers.