A Medicare Supplement Insurance policy, sold by private companies, can help pay some of the health care costs that Original Medicare doesn't cover, like copayments, coinsurance, and deductibles. Medicare Supplement Insurance policies are also called Medigap policies.
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 it's share of the Medicare-approved amount for covered health care costs. Then, your Medigap policy pays its share. You have to pay the premiums for a Medigap policy.
Every Medigap policy must follow federal and state laws designed to protect you and it must be clearly identifies as "Medicare Supplement Insurance." Insurance companies can sell you only a "standardized" policy identified in most states by letters A-N. All policies offer the same basic benefits, but some offer additional benefits so you can choose which one meets your needs.
Source: 2019 Medicare & You, The Official U.S. Government Medicare Handbook