A content delivery network, or CDN, is a network of servers spread across different geographic locations that all keep copies of a site's static files, images, CSS, JavaScript, so a visitor's request gets served from whichever server sits physically closest to them instead of always traveling to the site's main server.
Distance adds real, measurable latency: a visitor in Singapore requesting a page from a server in Virginia waits for that round trip regardless of how fast the server itself responds. A CDN cuts that distance down to whichever edge location is nearest, which speeds up load times and takes load off the origin server during traffic spikes, since cached content doesn't need the main server at all. Most CDNs also add a layer of protection against traffic surges and certain kinds of attacks as a side effect of sitting in front of the origin. For any site with visitors spread across regions, a CDN is one of the more reliable ways to improve page speed without touching the site's own code.
