The Login pop-up is a floating tag that follows a guest visiting your forum inviting him to either identify himself and login if (s)he is already or register if this isn't the case yet
The tab contains by default all the elements allowing a quick connexion for the member or a quick registration for the guest.
To activate this feature, go through: Admin Panel => General => Configuration => Login pop-up => Activate => Yes
The admins will then be able to add a personal message to the pop-up.
The pop-up adopts the colors of your forum and shows up as soon as a guest arrives on any of the pages of your forum: