QT is short for quantitative tightening.
Quantitative tightening is a monetary policy where central banks reduce (sell) their holdings of government bonds and other financial assets to decrease the money supply and control inflation.
This process is the reverse of "quantitative easing," where central banks purchase assets to increase the money supply and stimulate economic growth.
QT is bullish for mortgage rates because, other things equal, it's associated with rising bond yields.