An annuity is a financial product that converts a lumpsum of money into a stream of regular income payments, typically paid monthly, quarterly, or annually, for a specified period or for the rest of your life, depending on the type of annuity you choose. It's essentially the reverse of saving up a corpus — instead of building money, you're converting an existing pool of money into a predictable income.
Annuities are most commonly encountered in the context of NPS, where a portion of your retirement corpus must compulsorily be used to purchase an annuity from an insurance company, which then pays you a monthly pension for life. Annuity rates and payout amounts depend on factors like your age, the corpus size, and the type of annuity chosen (such as with or without payouts continuing to a spouse after your death), so it's worth comparing options carefully before committing.