A single server far from a visitor must send every image and script across the globe, which is slow. A content delivery network stores copies of those files on machines spread around the world, so each visitor is served from a nearby location rather than the origin.
Beyond speed, this spreads load across many machines, so a sudden surge of traffic does not overwhelm one server. The origin still holds the authoritative copy; the network simply caches and refreshes the parts that rarely change, which is most of a typical page.