Medicare Supplements are additional health insurance policies that you can add to your Original Medicare benefits. Medicare Part A and Part B pay for a lot, but not all, of the cost for covered medical services and supplies.
So, because Original Medicare doesn’t offer sufficient coverage, you can purchase a Medicare Supplement plan (aka Medigap policy) from a private insurance company like Cigna, Aetna, Mutual of Omaha, Humana, etc. to help fill the gaps.