A payload, in payments technology, is the actual data carried inside a message between two systems, as distinct from the addressing and control information wrapped around it. When a merchant's server calls a payment provider's API to create a charge, the payload is the body of the request: the amount, currency, payment method, customer reference and any other fields the provider expects. The provider's reply has a payload too, giving the payment's identifier and status.
In a REST API the payload is usually written in JSON. Bank messages built on ISO 20022 carry an XML payload. A webhook notification delivers a payload describing an event, such as a completed payout, and the sender normally attaches a digital signature computed over the exact bytes so the receiver can confirm that it is genuine and unaltered. Payloads containing card numbers or personal data are protected with encryption and should not be written to logs in readable form.