ActiveMQ is an open-source message broker that facilitates communication between different applications by sending messages between them. It supports various messaging protocols, allowing different systems to communicate seamlessly, which is essential in distributed systems. With features like message persistence, load balancing, and high availability, ActiveMQ ensures that messages are reliably delivered even in case of failures. It is widely used in enterprise applications to improve scalability and decouple components, making it easier to manage and develop complex systems.