CDN stands for Content Delivery Network. They automatically create copies of your files and distributing them on servers around the world. When someone visits your website, the file will be served from the nearest server, cutting down load times. In WordPress, plugins like Optimole, Bunny, and Rocket have capabilites to upload your images to a CDN, and in turn update the image links so they download for the visitor from the CDN, not your main WordPress server.