The document discusses the Message Queuing Telemetry Transport (MQTT) message format. It describes the core elements of MQTT including clients, brokers, topics, and subscriptions. It then provides details on the format of different MQTT message types like CONNECT, PUBLISH, SUBSCRIBE, and DISCONNECT. This includes explaining fields like client ID, clean session flag, will message, message identifier, and payload. Examples are also provided to illustrate MQTT message flows between a client and broker.