My blood pressure has a normal systolic yet a high diastolic. What does this mean?

Normal not = Healthy. No specific values were stated. By decades of human trials, NIH.gov, since 2003, officially considers systolic BP 120-139 as prehypertensive, diastolic BP 80-89 as prehypertensive. What do they mean by prehypertensive? High BP they are unsure is bad enough to tell Docs to Rx BP meds because many/most do not have a good long term track record in human trials, only a few do. Optimal rest PP 40mmHg.
Not unusual. That's a typical hypertensive pattern in younger people. The elderly tend to get primarily systolic hypertension while younger people tend to get diastolic hypertension. It's still hypertension, still damaging, and still needs to be treated.