hertz

"Hertz" is a word or more accurately a "unit" used to measure the number of waves that pass you in 1 sec, say when you stand in an ocean on a beach.

If you say 20 hertz, then you are telling someone that there are 20 waves that passed you in 1 sec. It is unlikely that the ocean waves are that frequent but sound waves can be that frequent and all the way to 20,000 hertz or 20,000 waves per sec. Light waves are billions of waves every sec and they are typically measured in Giga Hertz. A giga is a billion like a kilo is a thousand.