Blood pressure is measured with an instrument called a sphygmomanometer.
First, a cuff is placed around your arm and inflated with a pump until the circulation is cut off. A small valve slowly deflates the cuff, and the doctor measuring blood pressure uses a stethoscope, placed over your arm, to listen for the sound of blood pulsing through the arteries.
That first sound of rushing blood refers to the systolic blood pressure; once the sound fades, the second number indicates the diastolic pressure, the blood pressure of your heart at rest.
Blood pressure is measured in millimeters of mercury (mm Hg) and recorded with the systolic number first, followed by the diastolic number. For example, a normal blood pressure would be recorded as something under 120/80 mm Hg.