If you post a reply, then immediatley look at the list of replies, your reply will of course be listed at the top at that moment. Hence, "Bump to the Top", "BTTT," or "Bump"--signifying that the reply is for purposes of attracting attention to the thread, more than or to the exclusion of any substantive comment.
John Rob is announcing a way to call a thread to the attention of a pre-defined group of screen names of people whom the bumping individual hopes will be interested.