What are MQTT protocol publishers and subscribers?

The publisher and the subscriber are the agents that represent the beginning and the end of a communication with this protocol.

The publisher is the MQTT client who sends the message and the subscriber is the MQTT client who is receiving the message. However, these agents do not know anything about each other, nor do they communicate directly with each other under any circumstances.

In fact, they are not even aware of the other's existence. Since there is no such awareness on the part of these two agents, there is a third element that receives and sends messages to the correct recipient - this agent is called Broker.