What is meant by a guaranteed issue right pertaining to a medicap supplement policy?

This means that they cannot turn you down for issuing you a policy, no matter how sick you are or were. Another name of it might be Open Enrollment. They have to take you on.

In most cases, you have a guaranteed issue right when you have other health coverage that changes in some way, like when you lose the other health care coverage. In other cases, you have a "trial right" to try a Medicare Advantage Plan (Part C) and still buy a Medigap policy if you change your mind. (per medicare.gov)