It's a contract written by an insurance company, which guarantees income for the rest of your life in return for a set of payments up front. Note the "ann" in the word, which is also coincidentally the starting letters in annual - as in the payments happen annually. These payments either come in one plop or are "accumulated" over time. This type of payment is kind of an insurance policy. In theory, it has less risk vs. just buying a bunch of stocks. But there is a price to pay for that risk mitigation... usually in the form of lower returns to Annuitants.