Agents are people dedicated to answering customer queries about a product or service. They are the face and voice of the company and have a better understanding of customer needs than most other people in the company. Most companies have dedicated support agents but in small companies or startups, developers, product managers and sometimes, even the CEO double up as an agent.
Most cloud-based helpdesks are priced based on the number of agents using them. They can also be called support representatives or, less romantically, seats.