So what? What does it mean to be an employee at will? It means that you can be fired at any time, for any reason or no reason, with notice or without notice. It means, as a legal matter, that your job hangs by the barest thread, subject to being snipped at any moment, with no recourse. It means, as a practical matter, that the employer holds all the cards in the employment arrangement.
But there is one more element to the law of employment at will: yes, it’s true that an at-will employee may be fired for any reason or no reason. But even an employee at will may not be fired for an unlawful reason. The law puts in place some protections against dismissal even for at-will employees.