Medigap is Medicare Supplement Insurance that helps fill "gaps" in Original Medicare and is sold by private companies. Original Medicare pays for much, but not all, of the cost for covered health care services and supplies. A Medicare Supplement Insurance (Medigap) policy can help pay some of the remaining healthcare costs, like:

• Copayments
• Coinsurance
• Deductibles

Medigap policies generally don't cover: 

• Long-term care (like non-skilled care you get in a nursing home)
• Vision or dental services
• Hearing aids
• Eyeglasses
• Private-duty nursing

You can’t buy (and don't need) Medigap while you’re in a Medicare Advantage Plan. You can’t use Medigap to pay for any costs (copayments, deductibles, and premiums) you have under a Medicare Advantage Plan.