It's a subreddit/event that they do every year (or I think it's only once a year?) where every user can place a single, colored pixel on a large digital canvas.
Your pixel can be over-written by another user, and each user can only place one pixel every 5 minutes or so (and there's a lot of bot-use, which is lame)
So different subreddits will band together to try and orchestrate a picture and protect it from being over-written by other users (or bots... if possible)
So alliances between subreddits are formed, fall apart, some backstabbing and deception... all until the event is over and the final canvas is locked-in.
It was actually a pretty cool event. You could maybe join a discord, a pattern and placement would be followed, and you'd just drop a pixel where it needed to be when you could, to help the overall picture. Or go rogue and try and etch a tiny little corner somewhere yourself
I had fun with it last year, but c'est la vie. I ain't going back there for anything. Would be cool if the different instances could somehow do something of the same nature across the fediverse one day. It was the comradery and light-hearted fun that really made it
On reddit you go to /r/place and can place a pixel on a certain place on this huge canvas of like... idk 5000 * 5000 pixels wide? Its a huge canvas of pixels. So you and your friend or another subreddit can try team up and conquer part of the canvas, lets say the bottom right corner. And then you can all work together and create a little flag or another picture/image like this one. I am not sure on the limit of how many pixels each user can post per minute, or the exact number of pixels, but its roughly something like 5000-10000 pixels wide and high
EDIT: This image you see would be a tiny portion of the entire canvas. You can tell because the resolution of 'Spez' for example is pretty bad, which means its only a few pixels wide. If it was taking up the entire canvas it would be perfect resolution and you wouldnt see janky edges
This is an example of the entire canvas at some point over the past few days. Its probably actually something like 50000x50000 pixels
It started as an April Fools event. Reddit used to do unique April 1 events. This one was popular. Makes people make accounts so they can have another pixel to place. Makes coalitions. You would see flags between two countries defending each other.
I've seen comments that this is just a desperate bid to increase engagement before the IPO. So while it would be funny to put the logo on, it's engagement nonetheless.
I haven't ever participated in it but its just a canvas you can draw stuff on but requires lots of people cooperating together. Could be fun to do with friends I guess, like a common thing is drawing your countries flag with fellow countrymen