A file share allows the directory it's associated with to be available from network interfaces. Think of your network as a long hallway.  As you cruise down the hallway, most doors are closed, but there are a few doors that are open (these are the file shares) and if you show the guard your pass and it's valid (this is your i5/OS user profile and password), you're allowed to enter the door off the hallway.  Sometimes only you can open the door and once opened, there's very little to see.  (This is an example of a file share for a directory that has no subdirectories and contains only objects you or your group is allowed to work with.)  However, on occasion you may enter a door that takes you through a vast labyrinth of rooms and other hallways with wide-open doors for all to walk through.  You may be amazed at the wealth contained in each of the rooms.  (This is an example of a file share that's been assigned to the root ('/') directory.  Once the root directory is shared, the QSYS.LIB file system is shared.  What does that mean?  That means that, assuming the user has sufficient i5/OS authority, all libraries are available through your network including the database files in those libraries.  Imagine the "wealth" of information stored in those files! 

File shares are often used to enable drive mapping.  In the Windows world, shares are often defined to enable drive mapping for file and document sharing.  The same can be implemented in the IFS.  Imagine what is available to you - and every user on the system, if you map a drive to root and the object authority of all libraries and files is at least *USE.  All database files are now available through a Windows Explorer session.