[classicman]

One site had this explanation

Like with most legal jargon, "employment at will" has both a formal definition and a practical one. In this case, the theory is easy. The basic definition of "employment at will" says the employer or the employee may end the working relationship at any time and for any reason without fearing legal action. This means the employer can fire or lay off the employee whenever they want. According to the theory, the employers do not have to explain why they fired their worker.

This definition also claims the employee may choose to quit his or her job at any time. Under such circumstances, the worker does not have to give the employer the reason for leaving his or her current position.

On its face, this is a simple law that should work for both the employer and the employee. Unfortunately in practice, "employment at will" is not so clear. While most states follow the formal definition, many lower courts have passed laws to cancel the employer's rights. All of these laws have created many exceptions to the formal definition, and employers must keep this in mind if they need to fire someone.