About Medicare Supplement & PDP Insurance (from Medicare.gov)
A Medicare supplement insurance (Medigap), sold by private companies, can help pay some of the health care costs that Original Medicare doesn’t cover, like copayments, coinsurance, and deductibles.
Some Medigap policies also offer coverage for services that Original Medicare doesn’t cover, like medical care when you travel outside the U.S. If you have Original Medicare and you buy a Medigap policy, Medicare will pay its share of the Medicare-approved amount for covered health care costs. Then your Medigap policy pays its share.
A Medigap policy is different from a Medicare Advantage Plan. Those plans are ways to get Medicare benefits, while a Medigap policy only supplements your Original Medicare benefits.