The engine is the mechanism that makes a car move in a similar way that a messaging platform makes software components work together.
The point of the analogy is to remind architects that in the environments where they are constrained by organization’s enterprise architecture the communication patterns and related infrastructure that glues their application may already be predefined. No matter how good the infrastructure products may be, blindly assuming that the people operating the infrastructure are able to configure it for your needs is a recipe for disaster. This is especially true for quality attributes of a system that require high volume of transactions.
In environments where you cannot control the communication infrastructure always design with a backup in mind.
Firebrand Architect on duty: CK