These are fixed annuities either deferred or immediate that provides benefits that are linked to an equity index. The value of this index could be tied to stock or some other equity index.
The index annuities are always built to showcase the performance of a well known index such as S&P 500, Russell 1000 Index or the S&P 100. Owners of index annuities are supposed to be able to track a popular index by participating in general market changes. The indexed credit interest using a formula based on changes to which the annuity is linked. This formula is what decides how the additional interest is calculated and credited. But how much you get and the time you get it depends on the characteristics your annuity.
Always remember that indexed annuities serve more those who sell them than those who buy them. All the same, an annuity can be a good choice of investment if done under favorable circumstances.