Your heart is a pump, pushing blood around the body through the blood vessels (arteries, veins and capillaries).
Every time the heart beats to push the blood around the body, it creates what is known as systolic pressure. As the heart muscle relaxes, diastolic pressure is generated.

When blood pressure is measured, two different figures are taken – the systolic and diastolic. An example is a blood pressure of 120/70, which simply means that the pressure in your arteries as the heart beats is 120, and 70 as it relaxes.
If one or both of these numbers is higher than normal, it is known as high blood pressure (or hypertension).