Central banks make key decissions regarding monetary policy.
The cost of money is controlled by the means of setting interest rates. Those are the rates at which commercial banks can borrow money from the central bank. It is a very delicate instrument - only minute changes either way can cool down or overheat the economy. Low interest rates make loans cheaper, thus increasing consumer spending and the investments made by companies. A higher interest rate, on the other hand, might tend to lock up liquidity in the financial system by making saving more attractive.