Medicare Supplements can have a six-month wait on pre-existing conditions (health problem or problems you have had before the date the insurance coverage begins), unless you have had at least six months of previous health coverage that can be considered "credible coverage".   If you have had less than six months of credible coverage, the six month wait on pre-existing conditions is reduced one month for each month of credible coverage.

You would definitely prefer your medicare supplement to not have a six month wait, since a health problem that was diagnosed or treated during the wait would not be covered.

What constitutes creditable coverage?  Generally speaking, any other health coverage you had prior to starting on Medicare supplement policy:  group health, individual health policy, medicare Part A or B, medicaid, Tricare, etc.