Currently, it means they will continue to let some small part of the securities the Fed hold on their balance sheet mature without immediately re-investing the

Currently, it means they will continue to let some small part of the securities the Fed hold on their balance sheet mature without immediately re-investing the proceeds. Outright sales of these securities are another option that they seem reluctant to do at this point. Either way this has the effect of reducing the amount of money in the system (the opposite of QE where they created money out of thin air to buy the securities in the first place).