Medicare does not pay for all of the costs of hospital and medical care.  Medigap is a supplemental health insurance policy that covers the gaps left by the standard Medicare benefits.   These policies are sold by private insurance companies and cover a range of co-insurance and deductible payments not covered by Medicare. 

The best time to buy a Medigap policy is during your Open Enrollment Period, or during a Guaranteed Issue Period. Agents and companies are required by State and federal law to sell you a plan during both of these periods. Purchasing a Medigap during one of these periods ensures that you can buy the best plan at the best price.

Each Medicare Supplemental insurance plan is standardized and the benefits are identical from one company to the next. This makes plan comparison easier than comparing general health insurance plans.  However, premiums may vary between carriers and you should compare the costs.