MQTT protocol is a communication protocol designed for widespread use of IoT devices. It is a protocol designed to carry the data that emerges when industrial (IIoT) and non-industrial IoT devices perform their duties and the messages you want to transmit to the remote IoT device.
MQTT (MQ Telemetry Transport) is a messaging protocol designed to transmit telemetry information with as few resources as possible. Although it works with the Server / Client architecture, it actually consists of clients that subscribe to the topics created for receiving and sending messages. It is designed for M2M and IoT systems and performs the functions of transmitting and receiving data with limited resources.
MQTT consists of a broker and clients that subscribe to topics created within that broker. If you want to collect data under the heading "Heat" for a topic, for example, if you make your IoT or M2M device subscribe to the "Heat" topic on the MQTT broker server, the temperature data collected by your device in your IoT or M2M system will now be collected under this heading.
It is a preferred protocol because it uses less resources in terms of bandwidth and data size requirements of the MQTT protocol. In case of problems that may occur while the messages are being transported between the publisher and the subscribers, the intermediary stores the messages and transmits them when the subscribers are back online.