This is simply an HTML front end to all the enabled shares. It provides basic file functions like file/folder delete and rename, folder creation and file upload. It's a very useful interface for those clients who can't browse the network share directly (like some PDAs), or if you've got the router configured to allow access from the WAN.
As already mentioned, all shares on the router can be configured for access via CIFS, NFS and FTP protocols. Whether or not shares appear in the web interface is also configurable.