High pressure would be pressure that his higher than the standard average pressure. Likewise, low pressure would be a pressure value that is lower than the standard average pressure. The pressure of an area can be measured using a barometer.
Pressure can be measured in the following units: torr, kPa (kilopascals), mmHg (millimeters of Mercury), or atm (standard atmosphere). The standard average pressure at sea level is equal to 760 torr, 101.325 kPa, 760 mmHg, or 1.0 atm.
Air pressure is due to the collision of air particles with a surface. Original barometers, or Torricellian barometers, were composed of a glass tube that was inverted into a dish of mercy. Measurements, in millimeters, were recorded on the glass tube. The air particles would collide with the mercury. Thus, the air particles would push down on the mercury. This, in turn, would cause the mercury to rise up inside the tube. The greater the air pressure, the higher thee mercury would rise into the tube. The average atmospheric pressure was 760 mmHg (or torr).