It’s essentially a thread on the forum where participants can leave comments/chat about their journey through the book/the tech. People not taking part in the book club can also post comments, ask questions about things someone has posted about it, etc
There’s no hard requirements other than us suggesting you make at least one post per chapter (even if it’s just to say you’ve completed the chapter or are looking forward to the next one) so that it sets a goal for you to get through the book as well as help motivate everyone else in the club to keep reading