It is an FTP Server for the PS3, with it, on a retail with your choice of "hack" to enable unsigned code it allows:
• Full Read/Write access to all of "dev_hdd0" (including save areas, games, VM etc)
• Full Read/Write access to "dev_flash2" and "dev_flash3"
• Full Read/Write access to connected usb devices
• Full Read access to "dev_flash"
• Full Read access to "dev_bdvd"
Connect with your choice of FTP client - Prefered is the old "ftp" from the command line, however Filezilla seems to work, allowing only one connection, and disabling PASV support.
Furthermore, the Server does not support some commands (like rename, of all things!) so make sure what you do counts!
Use the IP shown on the screen - make sure to have your net connection up and running before hand (wired is so much faster)