A Subscriber, in the context of a message broker or messaging system, is an entity or component that receives messages and acts upon them. While the Publisher generates messages and sends them to the message broker, the Subscriber is a part of the system that has subscribed to receive specific types of messages forwarded by the message broker.

Subscribers are recipients of messages who subscribe to particular channels, topics, or types of messages that are relevant to them. Once the message broker receives messages that match the subscription criteria of the Subscribers, it forwards those messages to the corresponding Subscribers, who can then process or respond to them.

For instance, in a messaging system, a Subscriber could be an application waiting for specific types of notifications. Once the message broker receives messages that meet those criteria, it forwards them to the respective application, which then processes or reacts to the received information.