If prices rise by a few percent each year, the same amount of money buys a little less as time passes. Cash left idle therefore loses purchasing power, even though the number in your account has not changed. Over many years this erosion can be significant, quietly shrinking the real value of savings that are not growing.
Investing is one way to try to stay ahead of inflation, because assets such as shares have historically grown faster than prices over long periods, although never in a smooth or guaranteed way. When you judge how well an investment has done, it is worth thinking in real terms, meaning after inflation, rather than only looking at the headline figure. A return that merely matches inflation has kept your purchasing power intact but has not actually made you better off.
This content is general information, not investment advice.