You will always have a Persistent Level, and you can have one or more sublevels that are always loaded or are streamed in with Level Streaming Volumes, Blueprints, or C++ code.
The Levels window shows all of these levels, enabling you to change which level is current (indicated in bold blue text), save one or more levels, and access Level Blueprints.
The current Level is the one that will be modified if changes are made in the Level Editor's viewport. This allows you to easily work on multiple maps, as long as they
are all writeable.
By right-clicking on the Persistent Level, you can perform operations like making it the current level, changing its visibility and lock status, and selecting all the Actors in that level.
Right-clicking on any sublevel gives a similar array of options, with additional entries for removing the sublevel and changing its streaming method.
Changing a level's visibility is for visualization purposes only and has no bearing on whether or not a level will stream into
the game when it is run. However, a level that is not visible here will not be affected if you rebuild the level, which can be a great timesaver if you have complicated levels.