The SmartFavorites system in Opus is like an "automatic favorites list". It attempts to learn which folders you do most of your work in, and display them in a list as if you had manually added them as favorites. Folders that Opus notices you use a lot are displayed in the drop-down Favorites menu, in a separate section from your manually added favorites folder. See the page on the Favorites list for more information about using this drop-down menu.
The SmartFavorites list can also be displayed in the Folder Tree - turn on the SmartFavorites item on the Folder Tree / Contents Preferences page.
You can enable or disable, as well as configure the behaviour of, the SmartFavorites system from the Favorites and Recent / SmartFavorites page in Preferences.