The sub as a whole is for "daily games", a genre that's existed in some form for decades (such as the daily crosswords in newspapers) but really became popular online starting with Wordle a few years ago. A new puzzle in the same basic format every day.

This specific post is about a game called Connections, where you're presented with 16 words/short phrases and have to group them into 4 groups of 4 based around a single connection. Often there will be distractor connections with 2, 3, or even 4 words that could fit into a category, but if you used that category it would be impossible to group the remaining 12 into three groups of 4. Or there may be 5 or 6 possible words in a correct category, in which case you must find which one(s) need to go in a different category in order to complete the puzzle as a whole.

There's also Categories and Conlextions which do exactly the same thing, if you want to play more of the same game, or you don't want to use a product by the New York Times (who publish Connections).