Employment “At Will” is an important doctrine that employers and employees alike need to know. In simple terms, it means that either the employer or the employee can terminate the employment relationship at any time for any reason, or for no reason. There is a BIG CAVEAT to this though: know that various laws may restrict your ability to fire someone for wrongful reasons, such as those reasons related to race or sex.
As an employer, you have a responsibility to make sure your at will policy is clearly outlined in your employee handbook and that you take the time to explain that policy to any new hire. One sure way to make sure you remain on the same page is to have the new employee sign a letter indicating that they understand and agree to the fact that they are an at will employee. It may save you some headaches down the line.