A content delivery network (CDN) is a network of servers that deliver content. The purpose of the CDN is to cache and more quickly serve static content based on the geographical location between the origin server (the server your site is hosted on) and the user making the request. This means when someone in Japan visits your website hosted in the United States, it will load just as fast as a website hosted in Japan.
Not all CDNs are the same, some have a zone URL and others have a custom URL. The difference between them can be seen when viewing the source code for your page
Whether you’re using a zone URL CDN or a custom URL CDN to cache the static elements of your website, a CDN can your click-through rates since slow loading websites are often abandoned before they load.
You can see the details here
We use CDN for WordPress Website Builder and many other products.