A wavelength is the distance between two peaks in a wave. And, if you are on the same wavelength with someone else, you are in sync and share the same perspective.
You can say, "My best friend and I are on the same wavelength," meaning you truly understand each other and tend to see the world in the same way. This meaning has been around since the 1920s, but originally wavelength had only its scientific definition, "the distance between two peaks of a wave." Wavelengths can be measured on anything that travels in waves, like sound, light, or water. The Old English root word is wagian, "to move back and forth."