1) Alan Greenspan was the chairman of the Federal Reserve Bank, the central monetary bank of the United States. 2) The "Greenspan put" is described well at https://en.wikipedia.org/wiki/Greenspan_put . Literally, a put is a kind of financial contract. The "Greenspan put" wasn't a contract, but it involved behavior that seemed to predictable that the financial world thought it could count on the Fed to act in ways that lowered their risk. 3) While he was Fed chairman, Greenspan's policy was widely admired and seen as creating a sort of golden age of finance. After 2008-2009, Greenspan's critics said that Fed policy fueled and failed to stop a bubble, whose collapse touched off the global financial crisis.

P.S. Most U.S. native speakers would not know what the "Greenspan put" was unless they were interested in investing and finance. People who subscribe to the Wall Street Journal or the Financial Times would probably be familiar with the term.

Hello Sasha, I must admit I had never heard the expression before but a little research shows that it is the monetary policy of a previous Chairman of the United States Federal Reserve Board, Alan Greenspan, in the period 1987-2000. Hope this helps Bob