Central bank digital currencies are essentially a new hybrid monetary/fiscal policy tool. Historically the Federal reserve and many other central banks function by buying and selling the debt of their sovereignty to influence interest rates. They pay for these securities by essentially creating money that didn't exist before and trading it to an institution for their debt security. For a long time this money has just been digital, when the Fed creates money to buy a bond from JP Morgan, all they do is to update a line in a database. So they already have a digital currency, but only for dealing with large institutions.
The idea of these new CBDCs is to setup a similar system but where the digital currency could be distributed to individual citizens. This would give the Fed a more powerful tool than it has ever had before because giving money directly to people is almost guaranteed to create real economic activity unlike adding to bank reserves(which is why we haven't seen meaningful inflation after 11 years of the Fed spamming reserve creation).
Some of this is lightly speculative but a detailed scheme which is almost exactly this was already proposed in a bill to congress, The Banking For All Act: https://www.congress.gov/bill/116th-congress/senate-bill/357...
I don't think it is hyperbolic to say that CBDCs are the "how" part of MMT. They take most of the logistical difficulties out of direct to citizen cash transfers and this is their primary goal.