The law, discovered by Daniel Bernoulli, which states that when air speeds up its pressure is reduced, and when the air slows down its pressure is increased.
Daniel Bernoulli explained that the faster the molecules within a fluid move, the less pressure they exert on objects around them. This applies to all fluids, including water, air and gases. For example, the water in a pond will exert more pressure on the pond's bottom, than a flowing stream with the same amount of water will exert on the streambed.
There is, in general, such a relation between pressure and velocity, that whenever the velocity of a fluid (such as air or water) is high, the pressure is low and vice versa.