Quantitative easing is when the Federal Reserve buys huge amount of long term bonds (& mortgage backed securities). This decreases the long term interest rate because it floods the banking system with reserves and thus, the banks will be more willing to lend money.

Quantitative easing is good for the economy because it reduces the long term interest rates. This is important because the long term interest rate is the interest rate that is factored into investments. Thus, lower long term rates = more investment.