To parallel the other comment with a wiki leak, the short version is the risk-free interest rate (e.g. treasury bills) minus the rate of inflation.

So if you have 6% inflation, and the return on e.g. the 1 year t bill is 4.75%, you have -1.25% "real" interest rates, because your money at the end of your year of investment will be worth ~1.23% less than it was at the beginning of the year.

I'm not sure I completely agree that the result is actually as bad as that, since stuffing your money in the mattress would put you -5.5% in the hole by comparison, but still.