Inflation measures how much the general price level has risen over a period, usually expressed as a year-on-year rate. In Spain it's officially calculated from the IPC (Consumer Price Index) published monthly by the INE (Spain's National Statistics Institute).
Why it affects you even if you don't "spend" that money
If your money is sitting in a current account earning 0%, it loses purchasing power every year at the pace of inflation: with €100 you'll be able to buy less a year from now than today. That's why the real return on an investment is the nominal return minus inflation.
Inflation vs. deflation
The opposite of inflation is deflation (a widespread drop in prices), which is also problematic because it discourages consumption and investment, as people wait for even lower prices in the future.