The Bernoulli Principle is the concept that claims that as the velocity of a fluid increases, its pressure decreases. Daniel Bernoulli was an 18th century scientist who discovered this principle.
The Bernoulli Principle applies to any fluid, even air. It is possible to see this occur very easily through a venturi tube, or one that is narrower at the middle than at the two ends. Passing a fluid through such a tube, it can be observed that it moves through the more narrow part faster. Since the molecules speed up, energy is used up and does not exert as much pressure. This is known as differential pressure, which means that the pressure at various points differs. That is why the Bernoulli Principle is also called Bernoulli's Law of Pressure Differential.