It's a sort of social network for readers. You can sign up, register your books, write reviews, and watch the accounts of people you know so you get notifications when they register another title or write another review... and/or follow authors, and if an author blog is attached to an author account, you can get notifications about updates to those, too.
There are various groups with forums, too, which I guess are a bit like the reading recommendations section here, only more themed? Though some seem to be book-club, like, "let'S read book X this month and discuss it", too. I'm not sure, I haven't spent much time in that section.