A Medicare Supplement plan, also known as “Medigap,” is a private health insurance policy that you can purchase to help cover out-of-pocket costs associated with Original Medicare, such as copayments, coinsurance, and deductibles, essentially filling the “gaps” left by your basic Medicare coverage;these plans are sold by private insurance companies and are standardized by letter (A-N) to indicate the level of benefits offered, with each letter representing a specific set of coverage options across different insurers; you must have both Medicare Part A and Part B to purchase a Medigap policy, and you pay a monthly premium in addition to your Medicare premiums to access these supplemental benefits.