One-liner
Your money's value is not fixed. Two forces move it, and one is almost always working against you. First, exchange rates: you can earn well in dollars, but if you spend in francs and the dollar weakens, you lost purchasing power. Second, inflation, which eats your purchasing power over time unless your assets at least keep pace with it.
Why it matters
You can look 'up' in your home currency and still be poorer in real, global terms, because the single home-currency number hides both the FX move and the inflation bite.
Analogy
Measuring your height with a ruler that is quietly shrinking. The number looks steady while everything around you grows taller.
The catch
Judging your wealth in one currency alone. If you earn, spend, and invest across several, you have to look at all of them, not just the one printed on your statement.