I was reading a description of a project on Github that is a Python-based content delivery network.
Why is it important that it uses a "reverse caching proxy" - and what does that mean in this context?
I think you have the question backwards. It would make more sense to ask "Why would it be important that a reverse caching proxy uses a CDN ?".
Typically you put a reverse caching proxy in front of a web server. All inbound requests go through the proxy which may or may not pass the request to the web server.
It's great for reducing the load on the web server by caching static or dynamic content, and for other purposes such as security, compression, etc.
In this case, it's useless to use this proxy if your application is already hosted on Google App Engine (and a violation of ToS as well). It's meant to be used in front of a web server hosted elsewhere, a practice also known as web server "acceleration".
Just to clarify, SymPullCDN is a reverse caching proxy, not a "Python-based content delivery network". The 'CDN' part of the SymPullCDN name refers to the CDN aspect of GAE (many datacenters around the world) which is a nice feature for a proxy.