What that means is, your employer can fire you "at will", for any reason or for no reason. Say your employer hires in a manager who takes a look at you, and thinks you look his wife's ex-husband, so he fires you. Can he legally do this? Yes he can! I once worked for a company for 5 years, when they hired in a manager who just wanted a team of his own people, so he threatened to fire me.
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(Disclosure - This is a paid post for which I will be compensated.)