While the topic might give you the impression that the article is about some grammatical crime scene, it is not the case. We usually associate agent with some mafia or CIA that has super abilities of detecting and combating evil. An agent noun is something very different.
The term agent in English grammar refers to someone who is doing something or is making an action. The doer or action maker is called an agent and so ultimately agent noun is a person that is doing something in the sentence.
Sara is sleeping.
Agent noun: Student
Agent noun: Reader
Agent noun: Prosecutor
The employer fired his employee.