Air pressure is the pressure we experience due to the weight of the atmosphere. The typical air pressure at sea level is one atmosphere (1 atm, or 760 millimeters of mercury); as you climb higher, the air pressure decreases. For example, the air pressure on top of Mount Everest is less than the air pressure on top of Denali.
Air pressure is used by modern industry, in the form of pneumatic controls. Hydraulic controls are similar to pneumatic ones, but use fluid pressure instead of air pressure, and are more precise due to the less compressible quality of the fluids used.
A simple science experiment demonstrates the power of air pressure. Ask your parents for a few tablespoons of lemon juice or vinegar, a few tablespoons of baking soda, and an empty milk jug or wine bottle (preferably with the cork still in). Pour the ingredients into the jug and quickly cap it. You will see the cap start to loosen, or the cork fly out, when the chemical reaction inside the container generates enough carbon dioxide to raise the air pressure inside the bottle above the air pressure outside. This is the same principle that airlines use to pressurize the inside of an airplane's cabin. For an extra kick, ask your parents for a chunk of dry ice to put inside the milk jug. Dry ice, being made of solid carbon dioxide, greatly increases the air pressure when it "melts" into gas. Put the ice in the milk jug, quickly screw on the cap, and place the jug in a warm location to melt the dry ice. (The inside of an oven will do just fine... but be careful not to turn on the oven!)
The Bible teaches us that before the Flood, there was a "firmament", or vapor canopy, separating the waters of the heavens from the waters of the earth. Because all that water was pressing down on the Earth's atmosphere, the air pressure in Biblical times would have been several times greater than it is today. Some creation scientists credit the longevity of the patriarchs to the greatly increased air pressure — much as an astronaut living in a pressurized space station ages more slowly than his twin at home.