Blood pressure or Bp is a measurement of the force exerted by the blood against the blood vessel walls. This is most accurately measured with a blood pressure cuff and a stethascope. Bp is read as a fraction, with a numberator being the systolic pressure, (during the heart's contraction) and the denominator being diastolic (during the heart's relaxation stage).
Normal Bp is 120/80 or 90-140/60-90 ranges are all considered 'normal.'
Low Bp occurs when the pressure is below htese normal limits. This is usually not dangerous, except in extreme circustances, such as shock, blood loss, and blood volumn loss as with dehydration.
High Bp is usally more dangerous than low Bp. A high diastolic pressure (bottom number) is more important than a high systolic pressure as it is mostly during diastole that the heart recieves its own blood supply. So if the heart is not relaxing the blood supply will be low. High blood pressure can occur with partial blockage of the artieries. This blockage adds an increased work load on the heart and can damage the arteries due to excessive pressure.
As one ages the Bp will increase due to a life time of pressure exerted on the blood vessel walls and subsequent damage and repair that takes place on the artery walls. No need for alarm unless the Bp is consistently out of 'normal' healthy ranges.