A message broker is a software application that facilitates communication between different systems or services by managing the exchange of messages. It acts as an intermediary, receiving messages from a sender, processing them, and then forwarding them to the appropriate receiver. This helps decouple the systems, allowing them to operate independently and efficiently. Message brokers support various messaging patterns, such as publish/subscribe and point-to-point. They can also provide features like message queuing, routing, and transformation. Popular examples of message brokers include RabbitMQ, Apache Kafka, and ActiveMQ, which are widely used in modern software architectures to enhance scalability and reliability.