What Employment at Will Means
As are many employees only after the fact, you might be surprised to learn in advance that U.S. employers may legally fire you for just about any reason, no reason or even an unfair reason. That's partially because there are relatively few labor laws that protect workers from wrongful termination and none that generally protect from workplace "unfairness" per se. But it's more so because most states consider employment to be "at will" in legal jargon.
In plain English, the Employment At-Will Doctrine means that employment is presumed to be voluntary and indefinite for both employees and employers. As an at-will employee under the doctrine, you may quit your job whenever and for whatever reason you want, usually without consequence. In turn, at-will employers may terminate you whenever and for whatever reason they want, usually without consequence.
Either party may end the relationship without prior notice, but neither party may breach contracts. Employers cannot violate state or Federal laws, and generally cannot rightfully terminate employees who refuse to do something that is contrary to public policy and sound morality, such as breaking the law. But with these few exceptions aside, it's pretty much open season on employees year round.