Your doctor provides a medical service and bills Medicare.
Medicare pays the approved portion of the bills and sends the excess amount to Medigap.
Your Medigap plan pays the excess amount left over, according to the plan's terms.
"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."