See below. It is the pressure in your arteries. If it is too high, it can cause complications such as stroke. It can also be too low for a number or reasons such as bleeding or dehydration.
If its too high. It can eventually cause end organ damage & if too low can cause symptoms.
Can be harmful. Diastolic pressure is the lowest pressure reached inside blood vessels, and occurs as the hear is relaxing between beats. Normal values are below 80. Diastolic BP above 90 (or systolic above 140) will increase the risk of heart attack, stroke and kidney disease. It is definitely worth having BP checked regularly and treated if high.