The US Federal Reserve has two missions: keep inflation under control and have full employment.
What should be the desired rate of inflation? If it gets too low, that might be due to the economy stagnating and heading towards recession. Too high and people and businesses start hurting and indulging in more short-term spending as a hedge against future price rises. The magic number that the Fed and the economic pundits have settled on seems to be around 2%.
In its Statement on Longer-Run Goals and Monetary Policy Strategy (PDF), the Federal Open Market Committee (FOMC) judges that inflation of 2 percent over the longer run, as measured by the annual change in the price index for personal consumption expenditures, is most consistent with the Federal Reserve’s mandate for maximum employment and price stability. When households and businesses can reasonably expect inflation to remain low and stable, they are able to make sound decisions regarding saving, borrowing, and investment, which contribute to a well-functioning economy and the well-being of all Americans.