In MQTT, a topic is a named channel used to organize messages. Clients publish data to a topic, and other clients subscribe to that topic through a broker to receive the messages they are allowed to see. Topics are often arranged in a hierarchy, such as sensor/building1/floor2, which makes them useful for routing telemetry, alerts, and device commands.
Topics matter in cyber security because they can also be abused as covert communication paths. A malicious implant may subscribe to an unusual topic and wait for instructions, or publish stolen data to a topic that blends in with normal traffic. Defenders should baseline legitimate topic names, client identities, and broker addresses, then alert on unexpected topics, strange naming patterns, or MQTT use where the protocol has no business purpose. Careful authentication, authorization, and transport protection help keep topics from becoming an attacker-controlled control channel.