An indexed annuity, also known as a fixed-index annuity, is a type of annuity whose income payments are tied to a stock index, such as the S&P 500. Indexed annuities perform well when the financial markets perform well.An indexed annuity, also known as a fixed-index annuity, is a type of annuity whose income payments are tied to a stock index, such as the S&P 500. Indexed annuities perform well when the financial markets perform well.