Quantitative easing is a form of monetary policy, in which central bank purchases longer-term securities from the market. Central banks use it as a method to quickly increase the domestic money supply, boost economic activity, encourage lending and investment.
When central bank increases the money supply, it will have several short-term and long-term impacts on country's economy. For example, it will most probably create and inflation. In case of currency, the quantitative easing often tends to depreciate a country's exchange rates in relation to other currencies.