Guaranteed renewal means your insurance company must renew your policy when it expires, as long as you continue paying your premiums. They cannot refuse to renew your coverage because you got sick, filed a lot of claims, or developed a new health condition during the year.
Under the ACA, guaranteed renewal is a federal protection for all individual and group health plans. It works alongside guaranteed issue (insurers must sell you a plan) to ensure you can both get and keep coverage regardless of your health status.
Your insurer can only refuse renewal in limited circumstances: if you stop paying premiums, if you committed fraud, if the insurer stops offering that particular plan to everyone in your area, or if you move out of the plan's service area. They cannot single you out for non-renewal.
This protection was especially meaningful before the ACA, when insurers could, and frequently did, refuse to renew coverage for people who became too expensive. Someone diagnosed with cancer or a chronic condition might find their policy non-renewed at the end of the year, forcing them to seek new coverage with a pre-existing condition that could be excluded or priced out of reach.
Today, guaranteed renewal works alongside auto-reenrollment. If you have a Marketplace plan and don't actively shop during Open Enrollment, you're typically auto-reenrolled in the same or a similar plan. Your coverage continues without interruption.
No. Your insurer cannot cancel or refuse to renew your plan because of how much care you use. They can only cancel for non-payment of premiums, fraud, or if you lose eligibility for the plan.