A Medicare supplement (Medigap) insurance, sold by private companies, can help pay some of the health care costs that original Medicare does not cover, like copayments, coinsurance, and deductibles.
Some Medigap policies also offer coverage  for services that original Medicare does not 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 healthcare costs. Then, your Medigap policy pays it 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.