Blood pressure is a measure of the force exerted by blood on the walls of your blood vessels. It is made up of two measurements. The first is the pressure in your arteries (the blood vessels carrying blood to all parts of your body) when your heart contracts, and the second is the pressure when your heart is relaxed. Blood pressure is usually measured by a doctor or nurse using a cuff which is wrapped around your upper arm and then inflated and deflated. Your blood pressure depends on your age and size. Blood pressure can be low, normal or high. Low blood pressure rarely causes any harm (unless it is really low), but high blood pressure is a different story. This can cause all sorts of problems, including damage to your kidneys and heart. However, it is very unusual for children and teenagers to have any problems with their blood pressure, though there are some conditions which can cause abnormalities (such as kidney disease). So it is unlikely you will have your blood pressure measured, unless you are unwell for a particular reason.
Blood pressure is measured using a device called a sphygmomanometer