It stands for Bookface, which is a forum that Garry Tan created years ago for YC founders. It's a wonderful resource for the YC alumni network, but it's not an alternative to HN. Most YC alums are on both.

What's typical to see on Bookface are founders asking and answering questions about specific challenges, people launching their startups (that part is a bit overlappy with HN, but the discussions are smaller and I would say narrower), people looking to hire or get hired, people asking for intros, posts about apartments and offices for rent, announcements of events...that kind of thing.