Number in the list. So the first item in the list occurs roughly twice as often as the second item and roughly 9 times as often as the ninth item.

Disclaimer: I'm not an expert, I looked at the Wikipedia page in response to this story. https://en.wikipedia.org/wiki/Zipf's_law

The Ordinal of the position in the sorted list of observed values.

So basically if you order by frequency, order correlates with frequency? Isn't that just two ways of saying the same thing? Isn't it obvious? I don't get why we would need a law for that so I feel like I'm missing some detail.

Not just correlates.

rank * frequency = constant

Ah, that is the missing piece!