Blood pressure is the pressure blood exerts against the walls of the arteries. The amount of pressure depends upon the strength and the rate of the heart's contraction, the volume of blood in the circulatory system, and the elasticity of the arteries.
Two measurements of blood pressure are taken, the highest and lowest values for pressure, which correspond to the two main stages in the pumping action of the heart.

How is blood pressure measured?

Blood pressure is usually measured by an instrument called a sphygmomanometer.
To measure the pressure, an inflatable cuff, or wide band, is placed around the patient's upper arm and a stethoscope is applied to the artery just below the cuff.
By listening for changes in the sound of the pulse, the individual measuring the blood pressure knows how much to inflate the cuff, in order to stop blood from flowing in the arteries of the arm. 
Air is slowly let out of the cuff until the blood begins flowing again. At this stage, the sphygmomanometer records what is called the systolic blood pressure.
Additional air is let out of the cuff until the sounds become muffled. The instrument then indicates the diastolic pressure.
The systolic pressure corresponds to the contraction of the heart muscle, and the diastolic pressure corresponds to a relaxation of the heart.
The two pressures are expressed in the following way:

Systolic and diastolic blood pressure

systolic / diastolic—120/80

What is normal blood pressure?

A healthy (normal) blood pressure reading varies with age, activity, and altitude and from person to person.
Bearing in mind these qualifications, values between 100/60 and 140/90 are generally considered normal. A single blood pressure reading, unless very high or very low, should not be considered abnormal.
An average of several readings taken on different days is generally used.

What does high blood pressure mean?

High blood pressure, also called hypertension, is itself a major disorder that requires treatment. Untreated, this sustained rise in blood pressure can damage the heart, blood vessels, and kidneys.
Other serious disorders that cause blood pressure to rise well above the normal level include congestive heart failure and head injuries.

What causes low blood pressure?

Low blood pressure (hypotension) can result from shock and some diseases. It can cause fainting.