An indexed annuity (IA) is a contract between you and an insurance company. You pay premiums in a lump sum or periodically, and the issuer promises* to pay you some amount in the future. The IA issuer also provides a minimum guaranteed* interest rate on your premiums paid.
The first IAs that were introduced worked very simply; the interest rate was determined by computing the difference between the value of the index to which the annuity was linked on the annuity's issue date and the value of the same index on the annuity's maturity date. If the difference was negative (i.e., the market performed poorly and the value of the index decreased), interest was calculated using the minimum interest rate. If the difference was positive (i.e., the market performed well and the value of the index increased), the interest rate used was a percentage of the difference — but usually not the entire difference.