Air pressure refers to the weight or force exerted by air on any surface in contact with it. It is a measure of how hard the air molecules are pushing against that surface, and can vary depending on factors such as altitude, temperature, humidity, and wind speed. In general terms, high pressure means the air molecules are moving fast and colliding with each other frequently, while low pressure indicates slow-moving or less dense air molecules. Changes in air pressure can also affect weather patterns, and extreme changes can lead to storms or other atmospheric disturbances.