It's a subreddit where people can add pixels to a big blank canvas to make a community image. It's something Reddit only does every few years. People are using it to protest because it was put up in the middle of Spez actively destroying Reddit to get it ready to sell in the IPO.

This was in fact a stroke of genius on Reddits part because all those protesters who ran to log in and show their discontent did nothing but boost Reddits engagement figures and make it look like all the previous protests failed and that Reddit is more popular than ever.

If you're really upset about the changes to Reddit the best thing you can do is STOP USING REDDIT. People don't seem to care though as long as they get to graffiti a swear word onto a digital wall...