At-will employment means either the employer or the employee can end the employment relationship at any time, for any legal reason or no reason at all, without notice, unless a specific contract says otherwise.
It's the default employment relationship in nearly every U.S. state (Montana is the notable exception), and most startup offer letters explicitly state that employment is at-will, specifically to avoid any implication that a job is guaranteed for a set term.
At-will status doesn't override anti-discrimination and other employment protection laws. An employer still can't fire someone for an illegal reason, their race, a protected disability, retaliation for reporting a legal violation, just because the relationship is at-will. It removes the need for cause, not the legal limits on what counts as a valid reason.
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