Messaging is a very loosely coupled way to do system integration. Two or more applications can communicate via messaging without any of them knowing the name or the location of the others. Contrast this with email which requires that you at least know the recipient's email address.
Instead of delivering messages directly to a named recipient, the service integration bus sends messages to something that acts like a database. That something is called a destination. The destination could be an actual database, or just a flat file that acts like a database. The act of sending a message involves writing to that destination. The act of retrieving a message involves reading that destination.
The destination belongs to a messaging engine. The messaging engine belongs to a bus member. The message traffic on a Service Integration Bus consists of a bunch of network traffic among bus members, messaging engines, and ultimately, destinations.