The highest bit of the wave.
It's not an idiom. It means just what it says -- something is on the crest of a wave. The crest is the top of the wave.
The crest.
crest
The distance from crest to crest in a transverse wave is called a wavelength.
Wave Length
I assume you mean, the crests of a wave. That distance is called the wavelength.
In a wave, it is called the wave length.
Crest.
Brig y don (crest of the wave)
The crest of a wave is its highest point, as opposed to the trough of a wave, which is its lowest point.
constructive interference
The high point of a wave is its creast or peak.