When you travel through a low tunnel, you might be tempted to duck your head. Something that's low is short or shallow.
Things that are less than usual can be described with the adjective low. You might be low on gas in your car, for example, or wake up with a low amount of energy. A low ceiling can be dangerous for a very tall person, and a low voice can barely be heard. You can also talk about a gloomy mood as being low. And when cows moo, whether they're feeling low or not, you can cay that they low.