Ruby Version Manager (RVM) is a tool for managing Ruby installation on UNIX (including OS X) and Linux. This probably raises two questions in your mind: Why not just install Ruby directly? And if RVM is important, why doesn't this book say more about it?
It allows you to work with multiple versions of Ruby on the same machine. This makes it easier to deal with issues relating to a rapidly evolving language.
It helps you identify non-Ruby dependencies in your Gems.
These features are important to Ruby developers, but not to users who simply need access to the Ruby Gem that implements the AppFog command line tool. Because this book isn't about Ruby programming and because working with RVM can get very complicated, we won't cover RVM installation or use.
If you're interested in RVM, you can visit the website https://rvm.io/.