Lookup MSDN
http://msdn.microsoft.com/en-us/library/bb522469.aspx
OFFLINE
Sets the file offline and makes all objects in the filegroup inaccessible.
Caution note Caution
Use this option only when the file is corrupted and can be restored. A file set to OFFLINE can only be set online by restoring the file from backup. For more information about restoring a single file, see RESTORE (Transact-SQL).