Inflation is the slow, steady rise in prices over time — roughly 3% a year on average. It means the same dollar buys a little less each year, so cash that just sits there loses purchasing power even though the number on the bill never changes. Hide $10,000 in a safe for 20 years and it will buy what about $5,438 buys today. The bills are all still there. What changed is what they're worth.
At about 3% inflation a year — close to the long-run average — money loses roughly half its purchasing power over two decades. That sealed $10,000 still reads $10,000, but it spends like $5,438. Nobody robbed you; robbery would have been faster. Inflation just quietly ate the value and left no note.
You can't stop inflation, but you can outgrow it. Money that's invested tends to grow faster than prices rise, so its real buying power climbs instead of sinks. The same $10,000 invested at around 7% would grow to about $21,426 in real terms over 20 years — more than double, while the cash version shrank by half. That gap is the entire reason to invest rather than hoard.
No. A sensible cushion of cash — your emergency fund — is exactly right, because its job is to be there instantly, not to grow. The leak is piling extra cash far beyond that and calling it safe. Money meant to grow has to be invested just to keep pace with prices, let alone get ahead.