It's a theory that appears to be gaining ground among academic economists, in monetary policy circles, and... of course... among deficit-spending politicians.
MMT involves central banks creating new money to directly fund government budget deficits. It's practically the same as quantitative easing (QE)... at least when QE money is used to buy government bonds.
But MMT removes some of the smoke and mirrors. Instead of laundering the money through the bond market, it's handed straight to finance ministries to spend as they please.
There's nothing at all "modern" about this. Places like Argentina have been doing it for decades. In fact, the Argentine central bank only stopped doing it last year (most recently), as the country battles to reduce high inflation. Last year, consumer prices rose around 50%. This year, most people are expecting 30-40%.