A short link is like a shortcut for the web. A regular link to some specific content on the web is normally pretty long and sometimes unpractical to share. An URL shortener (service) handles that problem by wrapping the original long link in a short link package. When someone click the short link, the browser will first go to the URL shortener service and then bounce to the original link (target URL).
If you are able to edit the target URL, you have a dynamic short link, which is great for printed QR codes. Another benefit is that you can find out how many persons click your link, if you can't track the original link.