The Federal Reserve does manage the liquidity of capital, which is the amount that is available for investment. High liquidity means there is a lot of capital because interest rates are low, and capital is easily available. When credit is cheap, businesses and investors are more likely to borrow. The return on investment only has to be higher than the interest rate, so more investments look good. In this way, high liquidity spurs economic growth.
The Federal Reserve manages liquidity with monetary policy and the Fed funds rate. By lowering the Fed funds rate target, the Fed lowers all bank rates. This adds liquidity, which adds to the money supply. This has the same effect as printing money.
People say the Fed is printing money whenever it engages in expansive monetary policy. This can be a precursor to inflation. As cheap capital chases fewer and fewer good ventures -- or houses, or gold, or barrels of oil, or high tech companies -- then the prices of those assets increase. This leads to "irrational exuberance." Eventually, more of this capital is invested in bad ventures. As they go defunct, investors panic, and prices plummet, as they scramble madly to sell. That's what happened with mortgage-backed securities during the 2007 Banking Liquidity Crisis. This phase of the business cycle, known as contraction, usually leads to a recession.
The Fed then "prints money" to spur borrowing, investing and economic growth.