If a Treasury pays 4.5% and investors expect 2.5% inflation, the real yield is about 2%. It is the true cost of money, so it matters most for assets that pay nothing today, such as gold, growth stocks and Bitcoin. Holding those is easier to justify when cash earns little after inflation.
In crypto, the same words have a second meaning: "real yield" in DeFi means rewards paid from a protocol's actual fees rather than from newly created tokens.