It's a tool that allows you to share link lists easily. Say you have a bunch of links to online news articles to tweet. You can create a single short link that contains all the links to the new articles that you want to share.
Give your link list a title, paste your links and give each of them a short name if you want and then hit the button, voilà!
You will get a short link and also a password to modify the list in the future, should you wish to do so.