What does "Rack-based" mean in the context of ruby frameworks?

It means frameworks that implement the Rack interface, a common way for web servers and frameworks to communicate with each other. Using a Rack-based framework means that your web app can run on numerous servers (e.g., Mongrel, Litespeed, Passenger (Apache/Nginx), and whatever Heroku uses) without requiring special plug-ins.

Rack also provides "middleware," a lightweight layer between the web server and the framework. This lets you do things like proxies, logging, testing, and authentication in a fast and framework-independent way.