In short: Deflation is a general, lasting fall in prices: money buys more, but debts become heavier and people put off buying.
Simply put: It sounds good — everything gets cheaper. But if people expect things to be cheaper tomorrow, they delay purchases; businesses sell less, and wages and jobs fall too. A debt stays the same number while incomes shrink. Under the gold standard, deflations and bank panics were frequent. That is why the ECB sees inflation below 2% as just as unwelcome as inflation above it.
Example: You have a €10,000 loan and a €1,000 salary. If prices and wages fall by 10%, your salary becomes €900 while the debt stays €10,000 — it now weighs more.
Watch out: Myth: "Falling prices are always good news." In a deflation creditors and holders of money gain, while debtors and job seekers lose.
Source: Federal Reserve History, "The Great Depression" — https://www.federalreservehistory.org/essays/great-depression · ECB, monetary policy strategy — https://www.ecb.europa.eu/mopo/strategy/html/index.en.html