The official "hello-world" example for python on RabbitMQ's homepage describes a message broker in the following way:
RabbitMQ is a message broker: it accepts and forwards messages. You can think about it as a post office: when you put the mail that you want posting in a post box, you can be sure that Mr. or Ms. Mailperson will eventually deliver the mail to your recipient. In this analogy, RabbitMQ is a post box, a post office and a postman.
The major difference between RabbitMQ and the post office is that it doesn't deal with paper, instead it accepts, stores and forwards binary blobs of data ‒ messages.
For every registered application in Lime CRM, there exists a message queue (or a post box) in RabbitMQ.
Every time an object is created, updated or deleted in the platform, a message is sent to the application's message queue.
Communication to RabbitMQ goes through the amqp protocol over port 5672