This is supplemental health insurance provided by private insurance companies that covers some of the costs Medicare doesn't, including copays and deductibles. You don't need, nor can you buy, a Medicare Advantage Plan if you carry a Medigap policy.
To join a private Medigap plan, you pay a monthly fee to an insurance company in addition to the premium you pay to the government for Medicare Part B.
The costs of Medigap plans vary a lot, depending on the coverage, the company, your location, and your age. So make sure to shop around before buying one.
Programs vary from state to state. Be sure to find out what your state offers. You can get help from your State Health Insurance Assistance Program. Call 800-MEDICARE to find yours.