After reading the wiki for the privacy-focused Decentraleyes extension available for both the Chrome and Firefox browsers, I am still confused on what the extension does. According to said wiki, Decentraleyes works by emulating CDNs by serving common files locally, but what are CDNs? I understand that there are a number of CDNs providers out there, including Cloudflare, which not only help reduce latency but also do things like DDoS protection.
More specifically, if I also have the NoScript extension installed, how come leaving the cdnjs.cloudflare.com script blocked by default in the NoScript extension (a popular security extension) results in a functional site on some sites whereas on other sites you have to allow it in order to get a functional site? Don't websites typically use Cloudflare to hide the entire website behind their CDN to lower latency? And if I leave cdnjs.cloudflare.com blocked in NoScript, how come some websites are still accessible? Or is cdnjs.cloudflare.com only for JavaScript libraries, and not for hiding entire websites?
I'm just trying to understand what a CDN does. Is the term "CDN" really just a generic term for any general technology that allows websites to lower latency for any type of resource?