Short answer: The ACA’s medical loss ratio rule requires insurers to spend at least 80% of premiums (85% for large groups) on medical care rather than overhead and profit. If they spend too little, they must send the difference back as a rebate, which is why you might get a check.
Under the ACA’s medical loss ratio (MLR) rule, insurers in the individual and small-group markets must spend at least 80 cents of every premium dollar on claims and quality improvement (85 cents in the large-group market). If an insurer falls short over a three-year average, it owes the difference back to policyholders as an MLR rebate, paid out each year (typically in the fall). For employer plans the rebate may go to the employer, who then shares the employees’ portion; for individual policies it goes to the member. Getting a rebate simply means your insurer didn’t spend enough of your premiums on care.