"At will" employment is an employment relationship in which either party can break the relationship with no liability, provided there was no express contract for a definite term governing the employment relationship or a collective bargaining agreement between the employer and the employees (usually involving a Union), or discrimination protected by statute.
With "at will" employment any hiring is presumed to be "at will" and the employer may freely terminate individuals for good cause, bad cause, or no cause at all, and the employee is free to quit or cease work without notice.