Essentially, there is a massive pixel-divided canvas. It starts out pure-white.
You are allowed to place one single pixel of a certain subset of colors every x minutes (I don’t remember how often, somewhere between 5 minutes to an hour).
Your pixel(s) can be overridden by any other user who places their pixel on your space after you place yours.
Some people come together to form collaborative art, messages, etc. (As seen here).
In the past, there have been countries’ flags, a giant spreading black void, hidden Among Us beans, and many fandom-specific and subreddit-specific images.