An  annuity  is a long-term investment that is issued by an insurance company and is designed to help protect you from the risk of outliving your income. Through annuitization, your purchase payments (what you contribute) are converted into periodic payments that can last for life. 

As the name suggests, a  guaranteed annuity is an annuity with a  guaranteed  income. It provides you with the total security of knowing that your retirement income is  guaranteed  at a certain percentage of your accumulated fund.  Annuities  with a guaranteed  retirement income can do a lot to boost your pension income.

More info - https://www.gov.uk/government/publications/pension-benefits-with-a-guarantee-and-the-advice-requirement