A Medicare supplement plan, also known as a Medigap policy, is insurance you can buy from a private company that helps pay for certain out-of-pocket costs not covered by Original Medicare. It’s designed to fill in the gaps left by Medicare, including copayments, coinsurance, and deductibles. Because Medicare still pays first, you can still go to any provider that accepts Original Medicare. Medicare Supplement Plans come with a monthly premium and a Part D Prescription Drug Plan must be added at an additional monthly expense.