CDN is short term for content delivery network.
A content delivery network (CDN) is a system of distributed servers (network) that deliver web pages and other Web content to a user based on the geographic locations of the user, the origin of the webpage and a content delivery server ( Edge server).
Servers nearest to the website visitor respond to the request. The CDN copies the pages of a website to a network of servers that are dispersed at geographically different locations, caching the contents of the page. When a user requests a webpage that is part of a content delivery network, the CDN will redirect the request from the originating site's server to a server in the CDN that is closest to the user and deliver the cached content. The CDN will also communicate with the originating server to deliver any content that has not been previously cached.
The process of bouncing through a CDN is nearly transparent to the user. The only way a user would know if a CDN has been accessed is if the delivered URL is different than the URL that has been requested.