A Content Delivery Network, or CDN, is a way to increase the load speed of your website. It does this by duplicating your website's files and putting the copies in multiple servers across the globe. That way, one of your visitor's don't have to load the files from YOUR server but a server that is closer to them.
For example, if you live in California, and your server is there too, a visitor in New York will have to wait for your files to move across the country. But if you copy your files into a New York server, the visitor can load your files from that closer location.
The nice part is that a CDN is a group of servers but acts like one.