An other important feature which can be implemented in a metadata file system is the capability to state relations between object in the file system. For instance, if you have a pictures of your mom and dad, it would be possible to create a relation to there entry in your contact book.
So at this time you should have an idea of what a metadata file system is. You should notice that there are a big difference between a metadata file system (such as DBFS[1] and WinFS[11]) and search engines based on metadata (such as Strigi[17], Beagle[18], Gnome Storage[16] or Spotlight[19]).