Pressure is the weight of the atmosphere on the earth’s surface. This atmospheric pressure is lower at the top of mountains and highest at sea level. This is beacuse all the air above the sea pushes down on its' surface. Rapid drops in air pressure mean unstable conditions and a storm is on its way, while rises in pressure mean conditions are more stable and fair weather is returning.
Atmospheric pressure is measured with a barometer, which can be kept indoors. For more information on barometers, click here.