Medicare Supplement plans, also known as Medigap plans, are private health insurance policies designed to help fill the 'gaps' in Original Medicare (Parts A and B) coverage. These plans assist with out-of-pocket costs like deductibles, copayments, and coinsurance that Original Medicare does not cover. You must be enrolled in Original Medicare Parts A and B to purchase a Medigap plan, and they are sold by private insurance companies, offering standardized benefits across the nation, though premiums can vary.