Also known as Medigap, the Medicare Supplemental Insurance is a cover that fills the gaps left out by the original Medicare. It takes care of some aspects such as the Health care costs that are not catered for by the Medicare including the co-payments, co-insurance as well as the deductibles.
This means that American Seniors, when they use this plan, will get about 100% of coverage. It should not be confused with the Medicare Advantage plan, as it is not used to gain the benefits from Medicare, but used for getting the benefits that are not provided by Medicare Original Plan.