U.S. Government Gold Manipulation Document Declassified

Not sure what it means but in the 1970's, granted it was during a period of high inflation,the Federal Reserve, had a secret agreement with the German government whereby the German government agreed not to buy gold in the open market, or from other governments, at a price above the then-official U.S. government price of $42.22 per ounce, despite the fact that the open market price for gold was then trading between $160 to $175 per ounce.

Why? Why don't I understand the monetary system? And is gold something that we shoulld still be talking about? It's a common metal right?