A Content Delivery Network (CDN) is a globally distributed set of caching servers that serve static assets and (increasingly) dynamic content from a location physically close to each user, dramatically reducing latency.
CDNs work by pulling content from your origin once and then serving subsequent requests from edge nodes around the world. A user in Tokyo requesting a CSS file hosted in London gets it from a Tokyo edge node instead of crossing the Pacific.
Modern CDNs (Cloudflare, Fastly, AWS CloudFront, Vercel Edge) also run code at the edge — image resizing, A/B test routing, full HTML rendering — collapsing TTFB to under 100 ms globally.
Putting any public site behind a CDN is one of the cheapest, biggest performance wins available. It also provides DDoS protection, TLS termination, bot mitigation, and access logs — essentially free upgrades that come with the caching.
See it in the product: Performance monitoring.