Question on CDN

Hi, how does “serving the static media off a separate subdomain” help? Does different subdomain mean, having dedicated servers for this static content? and if yes, then what is the difference between having these separate servers v/s CDN?

“If the system we are building isn’t yet large enough to have its own CDN, we can ease a future transition by serving the static media off a separate subdomain (e.g. using a lightweight HTTP server like Nginx, and cut-over the DNS from your servers to a CDN later.”

I saw this content directly copied from here -
While I appreciate condensing the content into a course, I would appreciate adding some explanation to the content as opposed to directly copying text.