Monetary policy is the mechanism through which the supply of money is controlled by monetary authorities. The goal of almost every monetary authority around the world is stability of prices. If prices are unstable—either too high or, in some circumstances, decreasing—this causes unforeseen and unwanted consequences for the economy as a whole.
Monetary authorities usually enact changes to interest rates for the purpose of changing the demand for money. Monetary policy can be either expanding, when interest rates are lowered and more money is available at a cheaper, or contracting, when interest rates are raised to make money more expensive to slow price increases.