I explained this at the General forum already.
A media library holds all the files categorized. The user selects the songs on the playlist from the media library.
With hundreds or thousands of songs it is much easier to browse a media library than go through all the directories.
Mp3blaster already has groups so the easiest way to implement a media library is to make it possible to copy songs from one group to another. The user would have all his files in groups and then a group called "Current Playlist" with the option play current group enabled. As far as I know this is not possible yet.