This page is about the architecture components of a event driven architecture.
It's publish-subscribe messaging system.
A What is a broker in an event-driven architecture? receives messages (event) from publishers and delivers them to subscribers.
A publisher (a.k.a. producer) is an application that sends messages (event) to the broker.
A subscriber (a.k.a. consumer) is an application that connects to the broker, manifests an interest in a certain type of message, and leaves the connection open so the broker can push messages to them.
In a EDA architecture, the message is an event that defines a state change.
channels are the type of events and therefore define the expected data structure of the message.
They are known by the publisher so that it can subscribe.