A CDN (Content Delivery Network) is a network of globally distributed servers that stores copies of your site's static content (images, CSS, JavaScript) at points close to users. When someone visits your site, the files load from the nearest server, drastically reducing load time.
The SEO impact is direct: faster sites have better Core Web Vitals, especially LCP (Largest Contentful Paint). On top of that, Google crawls fast sites more frequently and efficiently. For sites with a geographically distributed audience, a CDN is practically mandatory.
The most popular options are: Cloudflare (with a generous free plan), Google Cloud CDN (integrated into the Google ecosystem), AWS CloudFront, and Fastly. For WordPress sites, Cloudflare with full-page caching can improve LCP by 50% or more without touching a single line of the site's code.