URL shortening is a technique that translates a long URL into an abbreviated/ shortened format.
Essentially, a URL-shortening service is a third party, serving up a link that sits between the email user and the end result. The long URL is converted to a short alphanumeric code. The success of this shortened URL (its viability) in the future depends on how reliable the URL shortener is.
However, URL shortening hides the final destination, so spammers extensively use shortened URLs to promote their nefarious links.