From Hydrogenaudio Knowledgebase
The Media Library feature allows foobar2000 to keep a list of your music files that can be quickly accessed using one of Media Library viewers.
 Music Folders
A list of folders containing your music files indexed by the Media Library feature. When no folders are present on this list, all Media Library related functionality is disabled.
Possible folder states:
- Indexing - folder is currently being scanned for music files and possibly monitored for changes depending on settings.
- Monitoring - folder is being monitored for changes.
- Idle - folder isn't being indexed or monitored.
- Pending - not yet added - apply preferences changes to add this folder to your library.
Note that files and folders marked as 'hidden' in your Media Library folders will not be indexed.
 File Types
These settings allow you to make Media Library selectively index only files matching some criteria within the folders you specified above.
For an example, the "exclude" option is commonly used to exclude cuesheets present in your album folders from being indexed, as shown on the screenshot.
 Installed Media Library Viewers
List of installed Media Library viewers, for your convenience. Most of them can be also accessed through menu commands and such.
 Library Viewer Selection Playlist
When enabled, this feature automatically maintains a playlist containing items currently selected in a Media Library viewer (such as Album List).
Note that the "Library Viewer Selection" playlist can be freely renamed without losing any of its functionality.