Here's how it works:
At the beginning of the game, you are asked what your name is, and whatever name you give the NPC mail deer (Ella), any letter you write will be signed with only the first letter of that name. The interface shows a character who is sitting in a bedroom writing letters, and there are basically three things you can do.
1. You can make a request about something you are struggling with, asking for advice, or just asking a question (some people make "smalltalk requests" like "How was your day?"). Other people can view your request and answer it if they choose.
2. You can view the pile of requests other users have made and answer them.
3. You can write a note of encouragement on a paper airplane, and send it flying through everyone's room.
You can also decorate your room, but this is not a main aspect of the game.
There's some explanation somewhere I think on the developer's website about how requests get sorted; basically they're sorted with the newest requests and requests with fewest replies on top, so those are the ones everyone sees first. For the most part, this seems to ensure that everybody gets multiple replies to their letters (though I don't know if that is the case for the occasional letters not written in English.)
It's also worth noting that when someone replies to your request, you cannot write any kind of reply back to them (as a safeguard against predators and trolls). However, everyone starts the game with one sticker, and you can send people stickers as a thank-you if they answer your request, and you can also put stickers on letters you write answering other people's requests.
Collecting stickers (which you can also use to decorate your room--when you use them to decorate, they turn into plushies, figurines, and things) is actually about the only game-like aspect of the game, and obviously, that's not the main point--the point is to say kind things to people. So, if someone wanted to say that Kind Words is "not really a game," I wouldn't argue with them. It's a "video game" in the sense that it is a video-based media, and it is interactive, so that just happens to be the most convenient label to say what it is.
That all said, this all comes across way easier by watching the game in action than by reading about it (if a picture's worth 1,000 words, is a video worth 10,000?), and there are plenty of YouTube videos out there reviewing the game. For example:https://www.youtube.com/watch?v=Ln2E4qn ... imeToGrind
This one is short and sweet, but still covers all the main aspects of the game.