To explain, your heart is a muscle, just like your bicep. Initially, this muscle can pump over higher pressures for a short time, as it does during times of exercise, stress, etc. If this high pressure is sustained, the heart pump will eventually fail and not be able to pump enough blood out of the heart and into the organs. On of the first and most dangerous organ to suffer from this reduced blood flow is the brain, in the occurrence of strokes. High blood pressure also contributes to the occurrence of heart attacks because the heart also delivers blood and oxygen to itself.
Hypertension can also cause damage to the retina and the kidney.
Treatment comes in two forms, lifestyle changes and prescription medication. Alcohol and salt should be restricted. There should also be no smoking and saturated fat should be decreased to lower the risk of complicating heart attack or stroke. If these changes in lifestyle do not reduce diastolic pressure below 90 or the systolic below 160, then certain medicines should be added. Medicine only helps if a person takes it.