Currently, central banks manage money supply by managing currency transfers to commercial banks and via managing the amount of cash in circulation. The commercial bank transfers are (almost) all digital. Cash, obviously, isn't.
There is reasonable reason to assume that cash will become less and less prevalent as digital payment flows become more ingrained. So central banks are looking at mechanisms to replace/supplement their retail money supply mechanism ie cash, with one that makes more sense for a more digital economy.
So, many central banks have been exploring digit fiat currencies, how they could be implemented, implications etc.