First and foremost, Supervisors are just that. That supervise, or over see day to day activities of an operation. A Supervisor is usually the boss. Their main function is to make sure that all operations are being done in a timely manner, and are being done correctly to insure quality, as well as safety.
Question is incomplete but it is best to assume that it is about the functions of supervisors. My answer is: First, and foremost, supervisors are in-charged of the workplace and they see to it that the business is running smoothly towards the accomplishment of the company's goals and objectives.