Blood pressure is really just that-the pressure at which blood moves around the body in your arteries. The easiest and least invasive way to test what that pressure is at any given moment is to momentarily stop the flow of blood and then slowly allow it to begin again. The pressure at which it begins to flow is the highest pressure the blood exerts on your artery walls.
Medical professionals do this by using a blood pressure meter known as a Sphygmomanometer (say that three times fast while eating peanut butter). They encircle a limb, usually an arm, with a balloon-like device known as a blood pressure cuff. While pumping the cuff up, they use a stethoscope to listen for your heart beat past where the cuff is cutting off blood flow. When they no longer hear your heart beating, they slowly release the pressure while watching the pressure gauge.
When they start to hear your heart beat again, this is the top number of your blood pressure, known as your Systolic pressure. They continue to release the pressure until they once again, no longer hear the heart beating, this is the bottom number of your blood pressure, known as your Diastolic pressure. Together these numbers tell them two things: the pressure that is inside your arteries between heartbeats (the bottom number) and the pressure inside your arteries when your heart squeezes (the top number). This information gives doctors the ability to assume several things about what is going on inside the body-things like how well the heart is working, is it working too hard or not hard enough, and is the pressure a factor in the symptoms a patient might be having?