Seriously, I would love to know the answer to this question, will someone please answer it!!!

What is quick back-and-forth motions repeated over and over?

That wasn't a answer, I NEED AN ANSWER!!!

wave? Pressure wave? Sound wave? Transverse wave?

I think they're thinking of vibrations. Maybe? My answer's just an educated guess from someone that knows very little about the science of sounds. So don't take my absolute word on it.

I do like how they said "repeated over and over." They had to repeat that it is repeated.