Here's a simple way to think about it:
* Imagine a slinky: If you push one end of a slinky forward, the compression (tightening) travels along the slinky. The coils of the slinky move back and forth in the same direction as the compression wave.
* Sound waves: Sound waves are compressional waves that travel through air, water, or solids. The molecules in these mediums vibrate back and forth in the same direction the sound wave is moving.
In contrast, in a transverse wave, the particles move perpendicular to the wave's direction. Think of a wave on a string or a ripple on water.
