Here's why:

* Ridges: These refer to the high points of the wave, often called crests.

* Move across the medium: This indicates that the wave is propagating, or traveling, through a substance (the medium).

Examples of waves with ridges:

* Water waves: The crests of the wave are the ridges that move across the surface of the water.

* Sound waves: While sound waves are longitudinal, they can be visualized as a series of compressions (high pressure areas) and rarefactions (low pressure areas). These pressure changes create the effect of "ridges" moving through the air.

Let me know if you have more information about the specific context, and I can give you a more precise answer!