QE is used when the policy rate is already near zero. It expands the central bank's balance sheet, lowers long-term yields, and tends to lift asset prices. It is the opposite of quantitative-tightening.
For traders, QE eras have been associated with low volatility and persistent bull markets, with the caveat that it is one factor among many.
Example: from March 2020 the Fed bought about $120 billion of bonds per month, expanding its balance sheet from $4 trillion to nearly $9 trillion by 2022.
Related: quantitative-tightening, fomc, federal-funds-rate, yield-curve