CDN is an acronym for Content Delivery Network.
Normally, when a page is requested from a web site, that file is copied from the website’s server, wherever it might be located, to the computer requesting that page, wherever it may be located.
A CDN is a network of servers around the country or planet. When used, a file is requested from the CDN instead of the website’s own server, and the CDN transparently delivers the file from the server that happens to be closest to the computer making the request. The net effect is that the file is delivered more quickly.