File gajim-remote.desktop appears on end-user system as a standalone application.
Menu launchers read file gajim-remote.desktop and produce an entry called Gajim Remote for end-users to see.
NoDisplay means "this application exists, but don't display it in the menus". This can be useful to e.g. associate this application with MIME types, so that it gets launched from a file manager (or other apps), without having a menu entry for it (there are tons of good reasons for this, including e.g. the netscape -remote, or kfmclient openURL kind of stuff).