Suppose you're designing a language course. All else being equal, you'd probably want to teach the most common words first, and there are a number of references you can check for that purpose. (For example, Wikipedia's page for the most common words in English.)
If the list includes the actual frequency counts (as this one does, for example) you discover something curious: The most common word ("the") appears about twice as often as the second most common, about three times as often as the third most common, and so on. Moreover, that holds true for pretty much any language. This is known as Zipf's Law after American linguist George Kingsley Zipf, who studied (but did not discover) the pattern.
But it isn't restricted to languages. The same general rule tends to hold true, at least approximately, for almost any similar sort of ranked list. It's related to the Pareto Principle or "80-20 rule" in economics (loosely speaking, 80 percent of the benefit comes from 20 percent of the effort).
The Vsauce YouTube channel has a quite interesting presentation on this here: