Annuities are contractually-executed, relatively low-risk investment products; the insured (usually, an individual) pays a life insurance company a lump-sum premium at the start of the contract. That money is to be paid back to the insured in fixed, incremental amounts, over some future time period (predetermined by the insured). The insurer invests the premium; the resulting profit/return on investment fund the payments received by the insured, and, compensate the insurer. 

Conventional annuity contracts provide a predictable, guaranteed stream of future income (e.g., for retirement) until the death(s) of the beneficiaries(s) named in the contract, or, until a future termination date – whichever occurs first. These financial instruments have been used to accumulate funds and provide significant and sudden increases in personal income (via future, lump-sum withdrawals), all while legally avoiding the taxes (e.g., income-, capital gains-, estate-) that would otherwise be assessed on them.