A network of globally distributed servers that cache copies of static content (images, video, JS, CSS) in locations close to users to reduce latency.
In AZ-900: Azure CDN, Azure Front Door. In CLF-C02 and SAA-C03: Amazon CloudFront. Question: what service serves static content with low latency globally? Answer: a CDN.
CDN (Content Delivery Network) is a concept that can show up on the exam. Check the full guide to see it in context.