An at-will employment agreement is a contract agreeing that you can be fired at any time, for any reason. In every state but Montana (which requires employers to have good cause to fire employees who have completed their probation period), employees are in fact presumed to work at will. The written agreement gives employers some extra protection if a fired employee sues. For more information on at-will agreements, including when you should think twice about signing one, see Nolo's article Employment At Will: What Does It Mean?
If you're an employer looking to draft a policy, see Nolo's Employer's At-Will Policy.