Word message defines content of information sent via message broker system.
As a message (package of data) we define any string of bytes, so it can be an information in text format (mostly readable for humans): in JSON format, simple messages or single lines of text e.g. server or application logs, and also data encoded in base64 format.
We can also have binary data e.g. file with a photo or a song in mp3 format.
The main feature of the message is that it passes through the queue system in an unchanged form. Additional information can be attached to the message, e.g. name, delivery time, etc. Only additional information can be modified, e.g. shifting the delivery time in case of an unsuccessful attempt.
In RabbitMQ — the same as in case normal parcel — additional information are stuck together, leaving its content unchanged.