A Medicare Supplement Insurance Plan (also known as a Medigap policy) is an insurance policy that helps supplement Original Medicare and is sold by private companies. A Medigap policy can help pay some of the remaining health care costs that Original Medicare doesn’t pay for covered services and supplies, like copayments, coinsurance, and deductibles. Original Medicare pays for much, but not all, of the cost for covered health care services and supplies. These are “gaps” in Medicare coverage.