Mass is the amount of stuff something is made of. Like water, air molecules flow, take different shapes and are affected by outside pressure. The same balance of forces that affects a boat affects an airplane. Air particles and molecules constantly bounce around and bump into each other, exerting a large amount of pressure and force. Everything on Earth weighs something because of gravity.
Gravity pulls the mass of a thing toward Earth.
At sea level, a cubic yard of air weights about 2 pounds. The rest is empty space. The air molecules in this room are moving at an average speed of about 500 meters per second or about 1,100 miles per hour. They collide with their neighbors about 7 billion times per second. A dash of other gases, including water vapor and carbon dioxide.