What is a CDN?

CDN is short for Content Delivery Network. As the name suggests, it is used to provide the content, content in the case of WordPress is the images, static files, scripts that load over a network. CDN helps to provide the content faster to an audience than directly serving plus has the benefit of reducing load.
Cloudflare is one of the popular and best CDN for WordPress, providing

• Security
• load balancing (as data served from CDN so the load is reduced on our site)
• SSL certificates
• Minification
  

and many other features, that help to make a site faster and at the same time protect from attacks.

The content delivery network is abbreviated as CDN. A content delivery network (CDN) is a network of dispersed servers that provide pages and other online material to users based on the user's geographic location, the webpage's origin, and the content delivery server.

If you go for free CDN then Cloudflare is a popular and effective CDN for WordPress.

CDN(content delivery network) is a service which helps us to deliver our content from nearest server & due to which our website speed increases.
CDN not only helps in improving website speed but also protect our website from spam & DDos attacks.
Popular free service Couldflare is one of the best services which most of website owners use.
Cloudflare also provides us free SSL(Secure socket layer) for website.