Due to file system permission restrictions on Linux and other Unix systems (and PHP Safe Mode restrictions), an FTP layer is used to handle file system manipulation and enable Joomla! installers.
Enter an FTP username and password with access to the Joomla! root directory. This will be the FTP account that handles all file system operations when Joomla! requires FTP access to complete a task.
For security reasons and if the option is available, it is best to create a separate FTP user account with access to the Joomla! installation only and not the entire Web server.
Note: If you are installing on a Windows Operating System, the FTP layer is not required.