If you are new to the term CDN, then let me tell you that its called “Content Delivery Network”
Content, the content is your static resources like css, images, js and smaller files
Delivery, how the content reaches to your website users
Network, how the near network plays an important role in the speed at which the content is delivered
CDN is a proxy that sits between your visitor (site user) and your origin (actual resource hosting) server. Whenever there is a request for a resource it goes via this proxy. The proxy is smart enough to detect if the resource is already there in its cache (edge server) or not? If yes it never makes request to the original hosting server and directly returns it from its own cache / copy.
Basically a CDN makes your site load faster, and this helps in SEO ranking and a better user experience.
A CDN will cache your content, and when requested, deliver it from the nearest location to the requesting user. The nearest location is one of their servers out of whole CDN server network.
They have cache servers all around the world, also called as Edge servers or PoPs (points of presence). When your site has a global audience, it is important that they get your site resources like images, css, js from the nearest location. Which will be faster, as compared to when loading them from a single server. This will also save your original server bandwidth.
To read more details please visit this KeyCDN article