CDN stands for Content Delivery Network. CDN stores copies of your website content (especially images, Javascript, and CSS files) and distributes them to your readers using a global network of servers. In this way, when readers visit your website, data will be loaded from the server closest to them. CDN often has very fast response and loading times, with the shortest downtime.
If you want to test CDN to see if it helps improve your network speed, please check out our best free CDN service summary for WordPress. This is a great starting point, without the need to invest a large amount of funds in advance.