What is CDN
A worldwide network of servers holding copies of a site's files, so visitors load pages from somewhere nearby.
In plain terms
A CDN keeps copies of a site's images, scripts, and pages on servers spread across the world. A visitor in Tokyo gets files from Tokyo instead of waiting on a server in Virginia. Cloudflare, AWS CloudFront, Fastly, and Akamai are the big names. Nearly every serious website sits behind one, and frontend performance work often means tuning it.
What it tells you about a developer
CDN knowledge signals a developer who thinks about users far from the server. It shows up in frontend performance work and in infrastructure roles. Someone who has tuned caching rules on a CDN has debugged why a page was slow for real users, which is practical, production-minded experience.