Every time you go to the doctor, they wrap a little inflatable cuff around your upper arm, pump air into it, and squeeze your poor little bicep until it just about explodes. They're measuring your blood pressure, but how does it work?
The air is pumped until your circulation is cut off (you know, that uncomfortable feeling). When they place a stethoscope under the cuff, they can't hear anything. Then as the air is slowly let out of the cuff, blood begins to flow again and they can hear your blood pulsing.
This is the point of greatest pressure (called Systolic), and is usually expressed as how high it forces a column of mercury to rise in a tube (that's why they look at that little dial). 120 mm is about normal.
Then, as more and more air is let out of the cuff, the pressure exerted by the cuff is so little that the sound of the blood pulsing against the artery walls subsides and there is silence again. This is the point of lowest pressure (called Diastolic). 80 mm is normal.
Normal blood pressure is 120/80 (systolic/diastolic) or less. When both numbers start to go way up, you've got high blood pressure.