The easiest way to explain this if you have no clue how fediverse works is - imagine there is a third party reddit, discord, X or some other platform's client that lets you browse their website/content. The developer of that client made a list of users whose comments/posts are hidden in said client. In reality, the client does not have significant user base and this affects almost nothing. Some users might not see your comments, but that's really their problem more than mine/ours at this point.

The dev made it so that if you used his frontend it would download a secret list of instances, communities, people, and words that they hand curated because he didn't like them.

This had the effect of a ban list that the user was unaware of, but really it would be more accurate to call it a censor list. The dev was censoring the Fediverse.

He mostly censored things that were "left-wing" or "woke", and he seemed to hate human rights advocates dispite having many alts on a star trek instance.

Later he effected a ddos attack within his "goodbye" message, although he is still quite active in the Fediverse and uses many alts to defend his own actions. One of those alts probably already posted a shit take on this post.

If you got banned by Tesseract, your posts would not be visible to anyone using Tesseract as their frontend. Even if you were not banned by wherever you are trying to post, explicitly blocked by the Tesseract user, or filtered out in some other way, your comments and posts would be hidden. And this hidden centralized blacklist was unilaterally administered by the dev of Tesseract.

Full disclosure: I got a regex on the blacklist 😁