QE is short for quantitative easing.
Quantitative easing is a monetary policy where central banks increase their holdings of government bonds and other financial assets (i.e., purchase them in the market) in order to increase the money supply and stimulate inflation.
This process is the reverse of "quantitative tightening," where central banks sell assets to decrease the money supply and curb economic growth.
QE is bearish for mortgage rates because, other things equal, it's associated with falling bond yields.