A Content Delivery Network (CDN) is a group of servers located around the world to improve website load times. The purpose of CDN is to pre-distribute content closer to the end user.
When a user requests a video, image, HTML page, etc, the data doesn’t have to travel from the original web host if there is a CDN server close by and ready to deliver it. This also mitigates the threat of DDoS attacks.
Ad providers leverage CDNs for snap content delivery, pre-cashing the content on the servers relatively close to the target audience.