If you're currently enrolled and take no action, you'll be renewed onto the same coverage for the following year. Prices change every year, though, so it's worth reviewing your dashboard to see how next year's costs and health budget compare.
If your plan is ending, you'll be prompted to shop for new coverage. Your carrier may move you to an equivalent plan, but not always — if there's no equivalent, you won't have coverage next year unless you actively choose a plan. After that you'd need a qualifying life event to enroll mid-year.