Here's why:

* Wavelength: The distance between two consecutive crests (or troughs) of a wave.

* Vibration: A complete back-and-forth motion of a particle in a wave.

Imagine a rope tied to a post. When you shake the rope, you create a wave. The distance between two consecutive peaks of the wave is the wavelength. To create that distance, the rope has to move back and forth once, completing a full vibration.