An agent is an employee of your company who has the rights to edit tickets and customers in your system. An agent is therefore an "internal" employee who has access to the content of your Zammad instance and can read or even edit it.
Every user who is supposed to view or edit tickets must be registered as an agent in your system.
Still don't really know about the work of an agent?
You can find information about "What is a ticket" or "What does an agent do" in our user documentation.