The first thing will be to see what a URL is.
URL is the acronym for Uniform Resource Locator and is composed of a set of characters that are intended to define a web resource (post, image or any type of content) so that it can be located.
It is what we all know as a link.
A URL shortener is a service that facilitates the action of shortening that URL, giving it new characters and shortening the link to make it more manageable and easy to use.
As an example we can see the URL of the Avada WordPress responsive theme, which is the following:
If we use a program like T2M URL Shortener to shorten or share, in this case, the same link becomes: