MQTT is an industry-standard IoT (Message Queue Telemetry Transport) protocol. It is a communication protocol for publish/subscribe (pub/sub).
The model for publish/subscribe is event-driven. Signals are pushed to customers who subscribe to the subject. The Broker is the contact point. Clients are sending messages to the broker and the broker is getting messages out to subscribers.
Messages contain the topic in the response which is used by the broker to route information. This means subscribers do not need to know the sender, because all contact is made through texts. Messages are pushed to subscribers, therefore the broker must have an open TCP connection.
If the connection is interrupted, messages may be kept by the broker for later forwarding.
The outcome is that MQTT is an architecture that is highly scalable.