CDN is the acronym for Content Delivery Network. It is a network of servers that store a copy of the content of its websites in memory (cache) to be delivered to visitors based on their geographic location, conecting them to the closer server and reducing the data transfer time (latency).
The CDN will also communicate with the origin server in order to deliver any content that has not been previously stored in cache. Also, its use allows for better protection against DDoS attacks and helps maintain high availability, since its infrastructure utilizes different datacenters simultaneously.
Content Delivery Networks are used both for B2B interaction and end user content delivery.
Today, as more and more aspects of everyday life move online, content delivery networks are used to speed up static and dinamic content, mobile apps, e-commerce transactions, video, voice, games, etc.
To learn more read the article in this link about the 12 benefits of using a CDN.