A CDN (Content Delivery Network) is a network of servers spread around the world that deliver your site's files from the location closest to the user.
Without a CDN, every request goes to your single server — maybe in Virginia. A user in Tokyo waits for data to travel across the Pacific and back. That round trip adds real delay, especially for images, CSS, and JavaScript files that every page needs.
A CDN copies your static files (images, stylesheets, scripts, fonts) to servers in dozens of cities. When someone in Tokyo visits your site, they get files from a server in Tokyo — not Virginia. The page loads faster because the data travels a shorter distance.
When you deploy to Vercel, your static assets are automatically served through their CDN. You don't configure anything — it just happens. That's why a Vercel-deployed site feels fast from anywhere in the world.
CDNs work best for files that don't change per user — your CSS, your JavaScript bundle, your images. Dynamic content (like "Welcome back, Alice") still comes from your server. That's why frameworks like React Router use SSR for the dynamic parts and let the CDN handle the rest.