Many pages have hundreds of links on them, clicking on properties for each one may take some time, this does the whole page and every page that is opened on your browser automatically when it is turned on.
As Pete says nofollow means no indexing by google, ie if it was an internal page it will never get into the big search engines, also sites don't want 100's of outbound dofollow links, it was originally used for comments on blogs, ie to stop spammers, but nowadays is used for sculpting page rank amongst other things.