CDN
A worldwide network of servers holding copies of a site's files, so visitors load pages from somewhere nearby.
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.
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.