What does the topic of messages MQTT represent?
Each message is allocated to a topic, like the subject of a message. The publisher sends the message to an associated topic. The subscriber informs the broker that he wants to receive messages that have been identified with that topic. The broker distributes the message to one or more customers who have subscribed to that topic.
Topics are structured like a directory, separated by slashes, as we usually see in a file system. Using this type of structuring, you can create a user friendly and self-descriptive naming and hierarchy.