A listener is a part of the engine that can receive messages, and can be used with the message bus to subscribe to specific message types. A listener can be set up by extending the Listener class.
A listener is just made up of a unique ID (an unsigned 32-bit integer) and a public
OnMessage method which allows
messages to be sent to and processed by the listener
See the Message Bus page for more details of how the message bus uses listeners to subscribe to message types.