CDN stands for Content Delivery Network.
A CDN is a network of geographically distributed nodes/servers with the purpose of reducing the transit time it takes to serve multi-media content (Video, audio, images) to the end user. When an end user sends a request for a content e.g. video on demand (VoD), a latency-based algorithm is used to route the traffic through the most optimal node (usually the node that geographically closer to the user), or directs the traffic through the fastest network route to the requestor (route with lowest latency). Consider the analogy of ATM. Instead of going to a bank and waiting the long line, you can simply get money quickly and efficiently from the ATM that is either physically closest to you or the one that is easiest to reach.
A good real-life example is video streaming services (Video on Demand, e.g YouTube, Netflix, etc.). When you watch a Netflix movie, you essentially connect through a content delivery network (CDN). By accessing the video from the most optimal edge server, you can enjoy a smooth streaming experience with no buffering.
