Messaging, irrespective of the particular product or product group, is based on queues and queue managers. Queue managers manage queues that can store messages. Applications communicate with a local queue manager, and get or put messages to queues. If a message is put to a remote queue (a queue owned by another queue manager), the message is transmitted over connections to the remote queue manager. In this way, messages can hop through one or more intermediate queue managers before reaching their destination. The essence of messaging is to uncouple the sending application from the receiving application, queuing messages at intermediate points, if necessary.
For more details see How MQe works.