Short answer: A qualifying life event is a major life change; marriage, divorce, a new baby or adoption, losing other coverage, or moving; that opens a special enrollment period letting you add, drop, or change coverage outside open enrollment.
Qualifying life events let you change coverage when open enrollment isn’t running. Common QLEs include: getting married or divorced; having, adopting, or fostering a child; losing other coverage (a job, a spouse’s plan, Medicaid, or aging off a parent’s plan at 26); a permanent move that changes your plan options; and gaining citizenship or being released from incarceration. After a QLE you typically have a special enrollment window (often 30 days for employer plans, 60 days on the marketplace) to make changes, and you may need documentation. Loss of coverage because you didn’t pay premiums usually does not count.