Medicare Supplement Insurance is also known as "Medigap" insurance because it provides coverage for the "gaps" in Medicare. These "gaps" consist of a hospital deductible, co-insurance for doctor office visits, etc. Medicare Supplement insurance is standardized and there are 12 plans available called plans "A through L."
For example: A Plan G from one company will have identical benefits to a Plan G from all other companies.
Most important for you to know, is that for the first six months from when your Medicare Part B takes effect, there is an "open enrollment" window. During this time, you can purchase any Medicare Supplement plan you want without health underwriting. If you wait to purchase Medicare Supplement Insurance until after this window, you may have to answer health questions on the application and the insurance company may have the right to decline your application. So, timing is important.