A CDN (short for Content Delivery Network) is a worldwide network of computers that keep copies of your website’s files close to your visitors, so the site loads quickly no matter where in the world a person opens it.
Here is the problem it solves: if your site’s files sit on one computer in, say, a single country, a visitor on the other side of the planet has to “wait” while the data travels a long way. A CDN places copies in many cities at once, and each visitor is served from the nearest one — like a popular chain that has a shop in every town instead of one warehouse far away. This shortens the distance the data has to travel and takes load off the main Hosting.
A CDN often works together with Caching, because both are about handing over ready copies quickly instead of rebuilding them from scratch every time.