In physics, the motion of waves through or along a medium. A “wave” can be sound, electromagnetic, electrical or other type. The concept can even be applied to the motion of an earthquake. A “medium” can be air, a gas, a liquid or a solid. In the case of electromagnetic waves, propagation may occur in a vacuum as well as in material media.
Sound is made up of waves, which are created by vibrating objects and propagated through a medium (normally, the air) from its source to a listener.
It’s important to understand that propagation does not mean the wave “travels” through the medium in the manner that you travel from one point to another. A good way to visualize this is to imagine a crowd in a stadium doing, uh… “The wave.” The person who starts the wave does not travel the length of the stadium to the other end. Instead, he sets the wave in motion, which is picked up by the next person, who transfers it to the next, and so on. After moving, each person (depending on the level of beer consumption) returns to an “at rest” state. The same is true of sound waves: each air molecule is charged with energy and set into motion, which is then transferred to the next molecule, and ultimately to the molecules nearest to your ear.