Hey there!
Usually, there's some sort of force, whether it be a wave or an earthquake.
A medium is what that force travels through.
Here's an example.
In water, the water waves would be the force, and the water would be the medium.
But not all matter needs a medium.
For example, we have light. If you've ever heard the speed of light being 300,000 meters/second on a vacuum, that means empty, open space. It's what's classified as an electromagnetic wave, and those actually don't need a medium.
Hope this helps!
