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A publisher (or producer, in Kafka terms) posts an event (sends a request) to a broker. This event is a piece of data to transmit and it can be many different things (ie. a JSON object containing information about a transaction, details of a newly created object in a database, and so on).
In order to separate one type of event from another, a producer sends it to one of the channels (or topics) specified within a broker.
If another application needs events related to some topic, it subscribes to the needed channel. Once a publisher posts an event to that channel, the subscriber (or consumer) receives the event and can work with it as it is needed by a business logic:
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