A Medicare supplement (Medigap) insurance, sold by private companies, can help pay some of the healthcare costs that Original Medicare doesn't cover, like copayments, coinsurance and deductibles. Some policies also cover services like medical care when you travel outside the U.S.
If you have Original Medicare and purchase a Medigap policy, Medicare will pay its share of the Medicare-approved amount for covered healthcare costs, and your Medigap policy pays its share.