Window which track visited game objects, prefab etc. From this window you can select object which you can recently visited.
There are 2 buttons to select previous and next object, as well you can select any object from history.
Active object is bolded. If you select object from history list it font come to the begining of that list, but if you select any another object after that from project or from scene it will be appended to the top of the list and become active (bold).