CDN stands for the content delivery network. It is basically set of servers managing resources per geographic region. For example, if I access your website in England from the USA. The CDN will deliver the static assets in order to speed up the website delivery. Open Source CDN means the software infrastructure to setup the CDN on OS of the server is open source. And anyone can modify, edit the code. And also this means you can have better infrastructure without having to actively pay for the development.