When a device publishes a reading, the payload is the bytes that represent the reading itself; the topic name, QoS flags and TCP/IP headers around it are overhead. On constrained links the ratio matters: a LoRaWAN uplink may carry only a few dozen bytes, so a verbose JSON document will not fit and a packed binary layout or CBOR is used instead. On Wi-Fi or Ethernet readability often wins and JSON is fine.
Payload design is a contract between firmware and backend. Field order, units, scaling factors and version markers all need documenting, because a fleet in the field cannot be changed quickly. Schema-based formats such as Protocol Buffers make that contract explicit and generate the encoding code on both sides.
Part of the IoT Glossary. Spotted an error or a missing term? Tell us.