Medicare provides a whole lot of coverage, but it doesn't cover everything. So some people choose to buy a separate policy to provide coverage for the areas Medicare falls short on. This is known as Medigap insurance. You buy Medigap from a private insurance company.
You can also use your Medigap policy to cover expenses you have under Medicare, such as annual co-pays and deductibles.
Important note: If you opt for a Medicare Advantage Plan (Medicare Part C), any Medigap policy you have won't pay out. So if you decide to move into a Medicare Advantage Plan and you already have a Medigap policy, drop the Medigap.