A URL shortener, much like bit.ly, t.co, or any other variation. You copy in a long URL, and you get a shorter URL that you can send to others.
Main difference is that when someone navigates to the short URL, they will not immediately be taken to where it points to. Instead they will be presented with the link, they'll see the preview of the page, and optionally they'll be able to select which of the query arguments and fragments they want to use when navigating there.
No more rickrolling!