What is the difference between a single premium and a flexible premium payment options in a deferred annuity?

What is the difference between a single premium and a flexible premium payment options in a deferred annuity?

Deferred annuities may be purchased with either a single lump sum or a series of payments over time. Single premium deferred annuities (SPDAs) require only one payment at the time the contract is established, whereas flexible-premium deferred annuities allow the purchaser to pay in multiple installments.