Expansionary monetary policy is when a central bank uses its tools to stimulate the economy. That increases the money supply, lowers interest rates, and increases demand. It boosts economic growth. It lowers the value of the currency, thereby decreasing the exchange rate.

Buying bonds is a tool of expansionary monetary policy because buying bonds will lead to an increase in output. Buying bonds increases the money supply to decrease interest rates. When interest rates decrease, then output will increase.

Increasing the reserve requirement would be an example of contractionary monetary policy, not expansionary monetary policy.

Source- Article