each day there is a 5 letter word.
You have a limited number of guesses as to what word it is (iirc 6 guesses).
When you make a guess, it marks each letter with something indicating whether the actual word had that letter in that position, whether that word has a copy of that letter (and not one you already found), or whether that letter does not appear in that word.
All of your guesses have to be words.
In hard mode, all of your guesses have to contain all of the letters which you got right in a previous guess.
At the end (if you get the word within 6 guesses?) you are given an option to share (on twitter mostly, I think) a representation of your game, in a way that doesn't reveal what words you guessed or what the final word was, just which positions had which of the 3 markings, which, in this share feature, are represented using emoji with the colored square blocks.
This results in many people posting grids of colored square blocks, followed some fraction out of 6.
