What: Allow for the index rates on deposit to be negative.
Why: A negative interest can be charged on a deposit account, i.e. instead of receiving money on deposits, depositors must pay regularly to keep their money with the bank.
It's applicable if the market interest rate goes below zero: central bank (or private banks), in order to incentivize banks to lend money more freely and businesses and individuals to invest, lend, and spend money, rather than pay a fee to keep it safe.
This happened for example in Switzerland, Sweden or Denmark.