MQTT (Message Queuing Telemetry Transport) is a messaging protocol for the M2M or IoT communication of devices and applications.
In contrast to HTTP or CoAP, the request / response protocols, MQTT decouples all users of the communication. The data exchange via the sending of messages takes place via a central distributor (broker) to the respective participants.
The following link gives MQTT support and gives an overview of MQTT of the Shelly family:
http://shelly-api-docs.shelly.cloud/#mqtt-support
Note: The original contribution was created in German by SparkyMaster