RabbitMQ is a message broker / scheduler that allows sending/receiving messages.
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 the letter carrier will eventually deliver the mail to your recipient. In this analogy, RabbitMQ is a post box, a post office, and a letter carrier.
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.