A registered agent is a person who is registered with the state, and normally listed with the Secretary of State, who is authorized to receive, on behalf of a business, legal papers such as Service of Process (Subpoenas, Liens, Lawsuits, etc.), tax documents, annual reports, and official correspondence from the state and regulatory agencies.
The fact that the registered agent must be located in that state and available during normal business hours enables others to readily serve any claim upon the business.
Anyone can serve as the registered agent. An employee or officer of a company that maintains an office in the state often serves as the registered agent but most companies choose to have a professional registered agent.