WHAT is CDN - was {Re: https://www.debian.org/mirror/list debian.inode.at 403 Forbidden}

On 03/12/2021 08:49 PM, Paul Wise wrote:
> On Fri, Mar 12, 2021 at 4:54 PM mx vx wrote:
> 
>> The mirror debian.inode.at is not working any more.
> 
> Please report a bug against the 'mirrors' package in order to get it removed:
> 
> https://www.debian.org/Bugs/Reporting
> 
> Please note that Debian recommends using the CDN for downloading apt
> packages, especially for laptops and other devices that move around.
> 
> https://deb.debian.org/
> 

What is CDN? deb.debian.org gives no information.
How do I know if I have a "need to know"?