By this, he means the outright purchase by the government of private companies, or shares in those companies.
… money is injected into the economy through asset purchases by the Federal Reserve. To stimulate aggregate spending when short-term interest rates have reached zero, the Fed must expand the scale of its asset purchases or, possibly, expand the menu of assets that it buys.
… the government could increase spending on current goods and services or even acquire existing real or financial assets. If the Treasury issued debt to purchase private assets and the Fed then purchased an equal amount of Treasury debt with newly created money, the whole operation would be the economic equivalent of direct open-market operations in private assets.
Here, he suggests that, economically, the purchase of companies or shares in companies by the government is essentially the same as private individuals or companies choosing to make those investments. This is nothing short of astonishing, for a number of reasons, not the least of which is that the government could be buying equity, simply to get the numbers on the stock exchange moving upward, whereas, private investment is undertaken only when the investor believes that the company he is investing in is likely to prosper in the future. If the private sector is not buying a particular stock, there is a reason. Mr. Bernanke suggests that, for the government to invest the country’s money unwisely is somehow a means of creating a financial recovery.