An agent goes beyond a single reply. You give it a goal, and it plans and carries out the steps, looking things up, using tools, checking its work, to get there. It is the difference between an assistant who answers a question and one who goes off and completes the errand.
Agents are where a lot of AI is heading. You will meet them properly later in the course; for now, just know that "answering" and "acting" are two different modes.