CDN stands for Content Delivery Network or Content Distribution Network. It is a proxy service which uses a large distributed system of servers deployed in multiple data centres across the Internet. The main reason for using a CDN is to build up redundancy in your site - which means lowered loads on your servers and a quicker loading site for your users. Ezoic uses Amazon Cloudfront CDN.
For more information: http://en.wikipedia.org/wiki/Content_delivery_network.