Here's a breakdown:

* Propagation: This refers to the movement of the wave itself. Think of it like a ripple spreading across a pond. The wave doesn't move water from one side of the pond to the other; it just carries energy through the water.

* Medium: This is the substance the wave travels through. It could be water, air, a string, or even empty space (for electromagnetic waves).

* Velocity: This is a measure of how quickly the wave travels through the medium. It's usually expressed in units of meters per second (m/s).

Important Note: The velocity of a wave is *not* the same as the velocity of the particles that make up the medium. For example, in a water wave, the water molecules oscillate up and down, but they don't move horizontally with the wave.