These are the so called "In Chapter Bookmarks". If you're a member you can use them to keep track of the point where you had to stop reading.
By clicking on the "My latest Bookmark" link you'll be taken exactly to the point you left off the day before.
Even if you're not logged in, you can still use these links like normal browser bookmarks. Just click on one of the white boxes and bookmark the page that reloads. This way you can bookmark multiple paragraphs in multiple Fanfictions/Chapters.
This function is incredible useful if you want to point out a certain text passage to someone else. No need to read the entire chapter anymore.