It replaces the word "Wordle" with the total number of solutions remaining out of the original ~2,300 based on your currently guessed letters. If you open up the developer console, you can see the list of possible solutions left.

I usually use it after solving the day's word, to see how other openers may have done, or to see how close to the solution I could have gotten by a particular point.

I also made a video about my adventures (no spoilers) in poking under the hood of Wordle: https://youtu.be/AgharwWfh60