A CDN is a network of computers in different places that can send website files to visitors. It is like having copies of a popular book in several local libraries instead of making everyone travel to one faraway shelf.
Pictures and style files can be delivered from a nearby location, which may help a page load more quickly. The main website can still be managed in one place.
A CDN does not create the site's content; it helps deliver copies of some files. If a recently changed picture looks old, a saved copy may need to update.