Update on April 05, 2021
CDN is a "Content Distribution Network" and these are servers located in different locations to help reduce the loading time of websites. This network mainly benefits the speed of the sites, the bandwidth and the security.
The way it works is as follows:
A visitor from the United States accesses a website that is hosted on a server in Europe, without a CDN this user will experience high load times since the request has to travel from continent to continent for the server to confirm and return the content of the site.
Now the same situation but with a CDN, the same user will experience lower load times and this is because there is a set of servers (CDN) located at different points and they keep a cached version of the website content.
Here are some CDN providers: