It's a cloudflare/google captcha verification box that's suppose to appear to allow you to verify to post. Note that uhhhhh it will probably not appear for some reason. I saw it once and was able to verify but it like rarely appears from my tests.
The Cloudflare screen appears regularly for me. It changed how I move around the site and where I interact, because I’m tired of the interference going from one page to another, whether or not I’m already signed in. The checkbox usually pops up after several seconds each time, but sometimes it only refreshes and refreshes and… I give up.