Quantitative easing (QE) is an unconventional monetary policy tool used by central banks to stimulate the economy when conventional monetary policy is ineffective. It involves large-scale asset purchases by central banks, primarily government bonds, in order to increase the money supply and lower interest rates. The document discusses the history and implementation of QE by various central banks, including the Federal Reserve, European Central Bank, Bank of Japan, and Bank of England in response to economic crises like the Great Recession. It also analyzes the effects of QE on economies like the United States and Saudi Arabia through lower borrowing costs, higher asset prices, and increased economic activity.