This plastic bottle was sealed at approximately 14,000 feet altitude, and was crushed by the increase in atmospheric pressure (at 9,000 feet and 1,000 feet) as it was brought down towards sea level.
Atmospheric pressure is a force in an area pushed against a surface by the weight of the atmosphere of Earth, a layer of air. The air is not distributed evenly around the globe. It moves, and at different times, the layer of air is thicker in some places than in others. Where the layer of air is thicker, there is more air. Since there is more air, there is a higher pressure in that spot. Where the layer of air is thinner, there is a lower atmospheric pressure.
At higher altitude, the atmospheric density and pressure are lower. This is because high places do not have as much air above them, pushing down.
Barometers can be used to measure atmospheric pressure. There is the same atmospheric pressure from all directions. The SI unit for pressure is hPa. Other units such as Bar (unit) and torr are used for various applications.