The way url shorteners work is that I can set up http://bit.ly/2Ffns9i as a short url for http://brianodonovan.ie and then you can save yourself some typing by typing http://bit.ly/2Ffns9i into your address bar and be brought directly to http://brianodonovan.ie . In this case the number of keystrokes is hardly worth the effort, but in many cases URLs can be very long and hence difficult to type in correctly.
The danger of short URLs is that you don't know where they are going to bring you to. For example you might be expecting to be brought to https://www.mybank.com/myaccount but instead find yourself brought to http://www.hackers.ru/defraud-me
The way http://www.websiteplanet.com/webtools/redirected/ works is that you go there and type in your short URL and it tells you where you will be redirected to if you use that short URL (this might involve multiple redirects). You can then decide whether or not you feel safe to visit the site in question.
