When the amount of money in circulation equals the value of goods and services you have a stable currency value.
Inflation is when the money supply exceeds the value of goods and services so you have more cash with which to buy the same amount so the items cost more.
Deflation is when the amount of money reduces in relation to the supply of goods and services so the items cost less.
Inflation is called a hidden tax as the value of your savings reduces with inflation. For instance the value of the US $ has fallen by over 95% since the creation of the Federal Reserve in 1913.