Atmospheric pressure is a force in an area pushed against a surface by the weight of air in Earth's atmosphere. The earth is covered in a layer of air. However, this layer is not distributed evenly around the globe. 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.
The higher the altitude, the thinner the air is, and the lower the atmospheric pressure is. 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 atmospheric pressure is hPa. Other units are used for various applications.
Atmospheric pressure Facts for Kids. Kiddle Encyclopedia.