You can use your country's central bank inflation target (often around 2%), a long-run historical average for your country, or your own personal estimate. Actual future inflation is never known in advance.
This tool only models inflation eating away at money that sits still (cash, or a bank balance earning nothing). If your money is invested and earning a return, use the "real return" (return minus inflation) in an investment calculator instead to see net growth.
Because inflation compounds. A 3% annual inflation rate barely changes prices in year one, but over 30 years it roughly cuts purchasing power in half — the effect accelerates the longer the time horizon.