Medicare supplemental insurance policies are sold by private insurance companies to fill in some of the gaps in healthcare coverage that you will find in Medicare. They allow people to be nearly fully insured for their healthcare needs.
During open enrollment, your right to purchase a Medicare supplement policy is guaranteed, no matter your health condition or past medical history. Insurers cannot refuse to offer you a policy. You also cannot be asked to pay a higher premium because of insurance risks you may bring to the table. For example, a smoker will pay the same premiums as a non-smoker. There is no medical screening for applicants during the open enrollment.
If you miss the open enrollment, insurance providers can charge you a higher premium for pre-existing health conditions. They also can outright refuse to offer you insurance coverage at all if they choose to.
Open enrollment lasts for six months and it begins on the first day of the month in which you are 65 or older and enrolled in Medicare Part B.