A Content Delivery Network (CDN) is a global network of servers where local copies of a file are stored to increase the delivery speed of that file to nearby users. For example if a user is in Japan viewing a video that is hosted in the eastern US, it can be very slow to load that video. If a CDN is in use, the CDN would instead copy (or "cache") the video onto a server in or near Japan, so that it can be loaded much more quickly.
CDNs only cache a file after its been requested by a user, and only store the cached copy for a limited amount of time. This means delivering the file may still be slow the first time a file is requested, and after long periods where no user requests the file. However, CDNs do still help in this case by optimizing the route of the file from the origin (where the original file is hosted) to the edge (the server close to the user). CDNs can pick delivery routes that avoid internet congestion, similarly to how Google Maps helps you avoid traffic.
The decision of which server to load a file from happens dynamically behind the scenes, so the same URL can be used for users all around the world.
Common CDNs include Akamai, Fastly, Limelight, Level3, CloudFront, StackPath and many others.