t might help to think of a traveling fluid in terms of streamlines. These are imaginary lines that represent the path of fluid particles. Streamlines are far apart when the area surrounding the fluid is wide. But when the area becomes narrow, the streamlines are pushed together, decreasing the pressure in the fluid and increasing its speed.
We can apply the idea of fluid streamlines to all sorts of situations. Since both liquids and gases are fluids, we can apply Bernoulli's principle to things like air as well as water.