If you had/have a health problem prior to purchasing a Medicare Supplement, that particular problem is considered a pre-existing condition.  Prior to purchasing a Medigap policy, one should know if the pre-existing condition will be covered by the policy.  Many Medicare Supplements cover pre-existing conditions immediately.  Some do not.  An insurance company can accept an applicant and not cover the condition for up to no more than 6 months.  This a "pre-existing condition waiting period".   For this to be applied the condition must be either diagnosed or treated during the 6 months prior to the Medicare Supplement starting.  Medicare will still cover the condition, but the insurance policy will not.

Many companies that have a pre-existing condition waiting period will waive the waiting period if the insured has had prior credible coverage.  This means if you have been on an individual major medical policy, a group major medical policy, or Medicare Part A for example the insurance company will probably go ahead and waive the waiting period.  This is something an individual needs to have clarified by the agent.