Here's why:

* Longitudinal waves are waves where the particles of the medium vibrate parallel to the direction the wave travels. Think of a spring: when you push one end, the compression travels down the spring, with each coil moving back and forth in the same direction as the wave. Sound waves are another example of longitudinal waves.

* Transverse waves are waves where the particles of the medium vibrate perpendicular to the direction the wave travels. Imagine shaking a rope up and down. The wave travels along the rope, but the rope itself moves up and down. Light waves are an example of transverse waves.