In the United States, almost everyone is employed on what’s known as an at-will basis. That means an employer can fire an employee at any time for almost any legal reason or even no reason at all. It also means employees are free to quit at any time for any reason.
Managers need to understand at-will employment, especially if they have the authority to hire and fire employees. They can inadvertently destroy at-will status by implying that there is some sort of employment contract that protects an employee from being terminated.