The payment reference is a customizable* field used for bank transfers. Recipients of a Crypto-to-Fiat payment will see the payment reference in their bank account, helping with reconciliation (→ What does the recipient of a Crypto-to-Fiat payment see in their bank account?).
By default, Request Finance pre-fills the payment reference with the payer's name and a reference to what is being paid (e.g. "ACME Ltd, Invoice #123").