By you, personally
The forum software keeps personal records for each member/ user account.
Roughly speaking, all forums posts are marked as ‘read’ or ‘unread’ – for you personally, and by (at?) the current moment.
As well as for every other member, personally.
So, each forum member, when logged in, has its own personal ‘picture’ (in terms of colours and arrows)
When you log in, you can use the function ‘View posts since last visit’ and you will be given a list of threads that have at least one unread (by you) post, from the last your visit (= logging in, not just reading the threads as a guest).
All those threads with unread posts are visible (for you) being marked with that above orange dog-eared book.
You can also use the function ‘Mark all forums read’ - and all orange books (at the moment of calling the function) will become white (= marked as read).
While you’re online, all threads that are currently added by new posts (except your own posts! :)) will automatically marked with (by?) that orange-book-and-arrow.
If you log out, all unread posts will be automatically marked as if you have read them. So, the next time you log in, they will be visible (for you!) ‘in white’ (even though they are still unread by you).
Hope, Slava will correct me, if I made any mistakes in my ‘impromptu explanation’.