Short answer: A Special Enrollment Period lets you enroll in or change a Marketplace plan outside Open Enrollment after a qualifying life event, such as losing coverage, marriage, or a new baby. You usually have 60 days.
Outside Open Enrollment, you can only enroll in or change Marketplace plans if you qualify for a Special Enrollment Period based on certain life changes. The most common triggers fall into a few groups: changes in your household (marriage, a new baby, adoption or foster placement, or divorce that causes a loss of coverage), changes in residence (such as a permanent move to a new ZIP code or county), and loss of qualifying coverage (job-based, individual, Medicaid, or CHIP). Being offered an individual coverage HRA or a QSEHRA also opens an SEP.
Federal rules generally give you 60 days before or after the triggering event to choose a plan, and for a loss of coverage you can act in the 60 days before or after it ends. Other situations can also qualify, including becoming a U.S. citizen, leaving incarceration, or being affected by a federally declared disaster. When you apply, you attest that your information is true and may be asked to submit documents confirming the event.