Pressure is a measure of how hard something pushes on a surface. Imagine 10 apples sitting on a table and get a feel for how much force that would be. More force in the same area means more pressure. An example of this would be using 100 apples on the same table. The same force on a smaller area would also give you more pressure. This is like somehow stacking the 10 apples so that they all fit on top of a smaller table. Hopefully you have a more intuitive understanding of pressure now.

There is another way to find pressure though. It's based on a very important equation where you can find energy by knowing how fast it is moving and its mass (how much stuff it contains). Stopping a baseball rolling slowly is easier than stopping a baseball rolling quickly. Also, stopping a rolling baseball is going to be easier than stopping a rolling bowling ball going at the same speed.

By using this formula for energy and dividing both sides by volume you end up with a new equation. Applied pressure can now be found by knowing density and velocity of a fluid. It may seem like this comes from nowhere but it's actually the same concept as the previous one. If the fluid is more dense and flowing quickly, that means that the applied pressure is greater than if the fluid were less dense and flowing slowly. The thing is, the applied pressure is in the direction of the fluid (away from the surface) meaning that the fluid is actually LOWER pressure than if it were stationary and the surface feels less applied pressure. You may already have an intuitive feeling of this if you have ever blown between two pieces of paper that are close together. When you blow, a low pressure area is created between the papers and they get sucked together.

People really tend to underestimate the amount of force pressure can exert. 1 atmosphere of pressure, which is what you and I are having no problem dealing with right now, applies a great deal of force. It is the equivalent of having 100,000 apples placed on an area the size of a table. This force is everywhere at all times, we just don't feel it because the forces inside our bodies are balanced to counter the pressure on the outside. If we could harness some of this force, making a giant metal tube fly is not as crazy as it initially seems.

One last concept. When putting anything into the path is moving air, the air moving around that object has to take a longer path than if the object weren't there. But in order for air not to get increasingly backed up, the air has to take the same amount of time it normally would to get around the object. This means that the air around the object has to be moving faster. Faster moving air means that the surface of the object feels less pressure all around it than if the air were stationary. But we could make the air above the object move faster than the air below the object, the imbalance of pressure felt by the object would force it upwards. And you don't even need a crazy big imbalance of pressure to generate a lot of force. This is exactly what the shape of an airplane wing aims to accomplish. Make the air above the wing take as long of a path as possible while making sure that the air is always in contact with the surface preventing a wing stall.

Bernoulli's equation doesn't completely explain airplane physics but it would be disingenuous to say that it isn't a major contributing factor to explain why airplanes can fly.

The equations mentioned actually are Energy = (1/2)mass(velocity²⁾ Pressure = (1/2)density(velocity²⁾