A permalink is a link that allows you to go directly to the post in question. So on a blog, each post might move down on the main page, eventually disappearing into the archives, but a permalink would allow you to go directly to that blog post (and often only that blog post). On Fluther, it allows you to link someone directly to the comment you’d like them to see. The link will take them to the question page and move them down the browser to the appropriate comment.