If you do not provide this on the command line, you will be prompted for it. The address of the remote host. If you do not provide this on the command line you will be prompted for it. The directory you want to upload to on the remote host. If you do not provide this on the command line you will be prompted for it.
Variables are things like references to other html pages and the names of image files.
Gallery builds an html page by writing a piece of static html code to a file, adding the relevant variable, writing the next piece of static html code, adding the next variable, and so on until that html page is complete. Gallery allows you to change the pieces of static html code, thus changing the way your webpages look. These 12 files will contain your customised static html code from which gallery will create webpages if you use the -S option. The -S option must be followed by the name of the directory where your html scheme resides. This is useful, for example, if you want your html scheme to include graphics.