You mean Content Delivery Network?

Just ask your old pal Google. Although she probably refers you to her friend Wikipedia:
https://en.wikipedia.org/wiki/Content_delivery_network

CDN is a technology that transmits numerous kinds of content, like video, games, pictures, files via the internet in a rapid and stamble way. This service is effective in spending the delivery of content of websites with high traffic and websites that have global reach.

You use a CDN to help keep your website fast under heavy loads (high traffic) by diverting server load of your static resources (css, pictures, videos, etc) via another server (aka CDN service)

I wouldn't bother with a CDN until you are getting TONS of traffic. Wordpress can handle alot of traffic with just caching plugin.

A content delivery network is an interconnected system of cache servers that use geographical proximity as a criteria for delivering Web content.

​Using a content delivery network (CDN) can provide a better user experience with faster page and resource loading and less downtime. It can also save online businesses a lot of money and minimize damage from DoS attacks.

CDN (Contact Delivery Network) is a system of servers distributing throughout the world delivering web pages and other web content based on the user's geographiical location. CDN is important for SEO as it improves the speed and quality of content that is delivered to the user.