Nominal return shows how much the amount has grown, real return shows how much more you can buy with it. Real return is approximately equal to the difference between the nominal rate and the rate of price growth; for high values, a more precise calculation is used through the ratio of values.
Negative real return means savings lose value despite account balance growth. This condition can persist for extended periods and is recognized only when comparing the rate with inflation — the increase in amount itself creates the opposite impression.
For investments in foreign currency, the calculation is more complex: the rate and exchange rate change matter, as well as inflation in the currency in which future expenses are planned. A foreign currency deposit with a low rate may yield a negative real result if the currency strengthens.
Practical conclusion: compare instruments by real return after all costs. This answers whether savings bring you closer to your goal, while nominal rate only answers whether the account number is growing.