What it is
WARN applies to employers with 100 or more full-time employees and to layoff events above set thresholds. The notice goes to affected employees, the state dislocated-worker unit, and the local chief elected official. Liability runs per employee per day of shortfall, with capped damages and attorney fees.
What to do about it: The federal act has exceptions for closing plants and unpredictable layoffs. Whether your event is covered is a fact question: count employees, count days, and get the notice date in writing.
Plain-language description of US practice, not legal, tax, or financial advice. The paired guide carries the citations, dates, and thresholds; the state pages carry the local windows.