CDN stands for “Content Delivery Network“, which is a group of servers spread throughout the country or around the world. They replicate the data for a website in one country in another country. So you could call up a webpage from an Indian website but the server that delivers it to you would be located in your country, so it has less far to travel and loads faster. The CDN intelligently delivers the same content to people from the server that is closest to them.
This approach is great for large businesses because it allows them to avoid the kind of bottlenecks that would happen if they served data from just one location. A CDN also makes DDoS / DoS attacks less effective, because it naturally provides multiple targets. Downtime from things like technical problems can be virtually eliminated because the CDN can route traffic to working servers when something breaks. All
Cloud services and media distribution networks like YouTube or Netflix will often use CDNs to keep things running smoothly. Every user is connected to the best server for them, eliminating the need to choose the best location manually, which is what you need to do with some FTP services.
Note that although CDNs usually use standard URLs, you may see a tell-tale “cdn” in their web address.